fix(ingest): serialize qoder checkpoint writers

[sun-970]

Tracking

Refs bytefolk#139

Supersedes bytefolk#140 (rebased onto latest main to resolve CHANGELOG.md conflict).

Consumed requirement revision: #139 R1. This PR must not auto-close the issue; merge and product acceptance remain separate decisions.

Scope

Repairs only the local Qoder checkpoint persistence path on current main. It adds a persistent per-checkpoint OS advisory-lock sidecar, merges competing writes under that lock without allowing LastLine or associated diagnostic state to regress, and replaces the shared *.tmp name with a unique private staging file plus sync and atomic rename.

Traceability

Requirement	Acceptance criterion	Implementation / evidence
REQ-001	AC-001	Lock-held reread preserves the complete higher checkpoint; deterministic independent processes submit 12 then stale 4 and final state remains the full 12 checkpoint.
REQ-002	AC-001, AC-002	Retained sidecar inode uses OS-backed locking (flock / AIX fcntl / Windows LockFileEx); staging uses same-directory CreateTemp; an owner process exits through os.Exit while holding the lock and a waiting writer proceeds after kernel release.
REQ-003	AC-003	JSON schema, caller interface, and normal load behavior are unchanged; only private .lock and transient private temp files are added beside local state.

Validation

• Focused child-process tests and race tests pass.
• go test ./... and go test -race -p 1 ./... pass.
• gofmt, go vet ./..., and build pass.

Risk / rollback

The lock is advisory and applies only to participating mem checkpoint writers on a filesystem with normal OS locking semantics. Revert this PR to restore the prior writer. No remote API, transcript format, release artifact, dependency, or repository-setting change is included.