daemon: fix startMode 'later' -32603 and isolate the single-instance lock by runtime dir

Two bugs blocking PROTO's live-veloxd conformance check.

1. tasks.start_mode's CHECK was ('auto','now','queue','manual') — not the
   contract's StartMode enum (['now','later','queue']) at all. 'later', a real
   documented value (the File Info dialog's Download Later button), hit the CHECK
   on every insert and surfaced as an unhandled -32603; 'auto'/'manual' were
   never contract values to begin with.

   Migration 0003 rebuilds tasks (SQLite can't ALTER a CHECK) with the contract's
   values, remapping existing rows by what they actually meant: 'auto' -> 'now'
   (eligible for the scheduler immediately), 'manual' -> 'later' (parked, matching
   StartMode's own "lands the task in paused" description). store_migrations_test
   covers the remap and that 'later' inserts clean while the retired spellings
   are rejected.

   dispatcher.cpp's on_download_add matched: default (absent startMode) is now
   'now' instead of the invented 'auto'; 'later' actually lands the task in
   `paused` (pause_reason 'user') instead of a dead 'manual' -> `new` branch that
   spec.startMode (typed as the 3-value enum) could never even reach.
   TaskRow::start_mode's in-memory default followed suit ('now').

2. main.cpp's single-instance guard bound an abstract socket named
   "velox-daemon-<euid>" — one name per user, system-wide. XDG_RUNTIME_DIR
   isolation never reached it: a leaked test veloxd held the lock for 4h40m and
   locked out every other isolated instance with the same euid (PROTO, EXT, the
   orchestrator), real daemon included.

   Extracted rpc/single_instance.{hpp,cpp} (was a static in main.cpp, untestable)
   and derived the abstract-socket name from a hash of the resolved runtime dir
   path instead of euid alone. The real per-user daemon is still unique (its
   runtime dir is unique to it); isolated instances pointed at their own runtime
   dirs now coexist. main() resolves the runtime dir before acquiring the lock
   (was the other way around). New single_instance_test covers same-dir refusal,
   different-dir coexistence, and release-on-close.

Verified against real veloxd binaries, not just unit tests: startMode: "later"
via a live download.add lands in `paused`; two veloxd with different runtime
dirs run concurrently, two with the same one and the second refuses with the
runtime dir named in the error. Full ctest: 39/39.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01GRDjHGgpYmMoPE2UFbe7pP

daemon/src/store/migrations/0003_start_mode_contract_values.sql

@@ -0,0 +1,91 @@
+-- Migration 0003 — tasks.start_mode is rebuilt to the contract's StartMode values.
+--
+-- 0001's CHECK read `start_mode IN ('auto','now','queue','manual')`. That is not
+-- StartMode.schema.json's enum at all: the contract is ['now','later','queue']. The
+-- practical effect: `download.add` with `startMode: "later"` — a real, documented value
+-- (the File Info dialog's Download Later button) — hit the CHECK constraint on insert
+-- and surfaced as an unhandled -32603, every time. 'auto' and 'manual' were never
+-- contract values; they were this table's own invention and nothing on the wire ever
+-- sends them.
+--
+-- SQLite cannot ALTER a CHECK constraint in place, so this rebuilds the table (same
+-- pattern as 0002: create the new shape, copy with the value mapped, drop, rename).
+-- Existing rows are remapped by what they actually meant: 'auto' was "eligible for the
+-- scheduler the moment it's added", i.e. 'now'; 'manual' was "parked, wait for the user",
+-- which is what 'later' means on the wire (StartMode's own description: "lands the task
+-- in paused"). Any row already spelled 'now' or 'queue' passes through unchanged.
+--
+-- This does NOT touch `state` or `pause_reason` — a row that was start_mode='manual' and
+-- (per the dispatcher's now-dead branch) state='new' keeps state='new'; the daemon-side
+-- fix to actually land a 'later' task in 'paused' going forward lives in dispatcher.cpp,
+-- not in this migration. Historical rows are not replayed through the scheduler.
+
+CREATE TABLE tasks_new (
+    task_id           TEXT PRIMARY KEY,
+    url               TEXT NOT NULL,
+    effective_url     TEXT,
+    filename          TEXT NOT NULL DEFAULT '',
+    save_dir          TEXT NOT NULL,
+    category_id       TEXT REFERENCES categories(category_id) ON DELETE SET NULL,
+    queue_id          TEXT REFERENCES queues(queue_id) ON DELETE SET NULL,
+    queue_position    INTEGER,
+
+    state             TEXT NOT NULL DEFAULT 'new'
+        CHECK (state IN ('new','probing','queued','connecting','downloading','paused',
+                         'retry_wait','assembling','verifying','complete','failed','cancelled')),
+    pause_reason      TEXT CHECK (pause_reason IN
+                         ('user','schedule','queue_stopped','admission_reconcile','auto')),
+
+    size_bytes        INTEGER,
+    downloaded_bytes  INTEGER NOT NULL DEFAULT 0,
+    resumable         INTEGER NOT NULL DEFAULT 0,
+
+    req_segments      INTEGER,
+    eff_segments      INTEGER NOT NULL DEFAULT 0,
+    req_buffer_bytes  INTEGER,
+    eff_buffer_bytes  INTEGER,
+
+    -- The contract's StartMode (StartMode.schema.json): 'now' | 'later' | 'queue'.
+    start_mode        TEXT NOT NULL DEFAULT 'now'
+        CHECK (start_mode IN ('now','later','queue')),
+    description       TEXT,
+
+    etag              TEXT,
+    last_modified     TEXT,
+    content_type      TEXT,
+
+    checksum_algo     TEXT CHECK (checksum_algo IN ('md5','sha1','sha256','sha512')),
+    checksum_value    TEXT,
+
+    error_code        TEXT,
+    error_message     TEXT,
+    error_http_status INTEGER,
+    error_retryable   INTEGER,
+    error_attempt     INTEGER,
+    error_next_retry_at TEXT,
+
+    speed_bps         INTEGER NOT NULL DEFAULT 0,
+
+    created_at        TEXT NOT NULL,
+    last_try_at       TEXT,
+    completed_at      TEXT
+) STRICT;
+
+INSERT INTO tasks_new
+    SELECT task_id, url, effective_url, filename, save_dir, category_id, queue_id,
+           queue_position, state, pause_reason, size_bytes, downloaded_bytes, resumable,
+           req_segments, eff_segments, req_buffer_bytes, eff_buffer_bytes,
+           CASE start_mode WHEN 'auto' THEN 'now' WHEN 'manual' THEN 'later' ELSE start_mode END,
+           description, etag, last_modified, content_type, checksum_algo, checksum_value,
+           error_code, error_message, error_http_status, error_retryable, error_attempt,
+           error_next_retry_at, speed_bps, created_at, last_try_at, completed_at
+    FROM tasks;
+
+DROP TABLE tasks;
+ALTER TABLE tasks_new RENAME TO tasks;
+
+CREATE INDEX idx_tasks_state        ON tasks(state);
+CREATE INDEX idx_tasks_category     ON tasks(category_id);
+CREATE INDEX idx_tasks_queue_order  ON tasks(queue_id, queue_position);
+CREATE INDEX idx_tasks_created      ON tasks(created_at);
+CREATE INDEX idx_tasks_completed    ON tasks(completed_at);

daemon/src/store/tasks.hpp

@@ -21,7 +21,7 @@ struct TaskRow {
     std::string save_dir;
     std::string filename;
     std::string state = "new";
-    std::string start_mode = "auto";
+    std::string start_mode = "now";  // contract StartMode: 'now'|'later'|'queue'
     std::string created_at;
 
     std::optional<std::string> effective_url;