fix(filetail): preserve reconciled checkpoint on first open

[jmagar]

Problem

Post-merge Coverage for #154 exposed a file-tail first-open race. reconcile() persisted a start-at-end checkpoint, but the spawned task discarded the reconciled source snapshot and reloaded the registry before its first open. Under coverage load, the task could ingest a pre-existing line instead of starting from the checkpoint.

Failed main run: 30722500743 (Coverage).

Fix

• pass the exact checkpointed source snapshot from reconcile() into the task
• use that snapshot for the first open when the live source definition still matches
• continue reloading current registry state for retries
• preserve disable and reconfiguration behavior by validating the live definition first
• add a deterministic current-thread regression test

Verification

• original checkpoint race test: 50 consecutive passes
• deterministic first-open regression: 50 consecutive passes
• full file-tail suite: 37 passed, 0 failed
• cargo check --all-targets: passed
• strict cargo clippy --all-targets --all-features -- -D warnings: passed
• formatting, diff integrity, and module-size guard: passed

[Copilot]

Pull request overview

Fixes a file-tail “first open” race where reconcile() persisted a start-at-end checkpoint, but the spawned tail task could discard that reconciled snapshot and reload registry state before its first open—potentially ingesting pre-existing lines under coverage load.

Changes:

• Pass the checkpointed FileTailSource snapshot produced by reconcile() into the spawned task, and use it for the first open when the live registry definition still matches.
• Preserve existing disable/reconfigure behavior by validating the live registry entry first and continuing to reload registry state on retries.
• Add a deterministic current_thread regression test that reproduces the original timing hazard reliably.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/filetail/supervisor.rs	Ensures the first open uses the reconciled checkpoint snapshot (when the live definition matches), preventing pre-checkpoint ingestion.
src/filetail/supervisor_tests.rs	Adds a deterministic regression test that validates the first open honors the checkpoint snapshot created by reconcile().

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