Give audit, IDs and unstorable text their shared homes

[SaladDay]

Part of #1 (store layering), foundation PR 1d. Gives audit, identifier parsing and unstorable-text handling one home each, so domain adapters can use them inside their own transactions.

Audit

• writeaudit and adminaudit own the sources, validation, ErrInvalidSource and the read models: Reader, Filter, Page and ErrInvalidQuery.
• New persistence/postgres/auditpg:
  • RecordWriteAudit, RecordAdminMutation and RecordDeploymentMutation run on the caller's transaction queries and never begin or commit.
  • auditpg.Store serves the audit reads in one snapshot and runs retention.
• Behaviour is unchanged:
  • Administrator provenance takes precedence.
  • An unattributed write records nothing.
  • Malformed provenance fails closed.
  • An audit failure aborts the business write.
• api gets a separate AdminAudit dependency.
• Deleted from store: write_audit.go, admin_audit.go, admin_history.go and write_audit_queries.go.

IDs

• pgunit gets ErrInvalidID, ParseID, PathID and LookupCursor.
• api passes path IDs unchanged, and storage resolves them with PathID.
• store.UnknownResourceID, parsePathID and lookupCursor are deleted.

Shared errors

• textvalue.ErrUnstorable is paired with pgunit.IsUnstorableText.
• credentialcrypto.ErrUnavailable replaces store.ErrCredentialStorageUnavailable.
• api has one helper for each shared error: writeTextValueError (400), writeCredentialUnavailableError (503) and writeAuditSourceError (400).

Also

• pgunit.Pool.Queries() is added for single-statement pooled reads.
• cmd/server builds one units := pgunit.NewPool(pool).
• IMPLEMENTATION.md Layering names the new owners.

HTTP responses are unchanged, and a new test checks the audit error bodies byte for byte. The OpenAPI schema names moved from store.* to writeaudit.*/adminaudit.*; their shapes are identical.

Checks

Check	Result
go build ./..., go vet ./services/core/...	pass
api	1372 passed
cmd/...	116 passed
auditpg (PostgreSQL)	12 passed
pgunit and pgtest (PostgreSQL)	11 and 1 passed
execution/...	201 passed
sandbox/providers	433 passed
full store package	1008 passed, 33 opt-in skips
make openapi	no diff
scripts/check-names.py	pass

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