feat(oracle): harden parser coverage gates

[rebelice]

Summary

• Convert Oracle parser foundation to dual-return error propagation, harden lexer/parser error handling, and enforce Loc sentinel/location contracts.
• Add Oracle soft-fail and strictness coverage, parser progress gates, BNF/Loc/keyword coverage manifests, and Oracle 26ai reserved-word audit coverage.
• Add optional real Oracle reference validation through DSN or Oracle Free testcontainer, including V$RESERVED_WORDS keyword audit.

Current Coverage Accounting

• Parser methods: 445/445 return errors, with 0 bare parse methods and 0 silent parser error discards.
• Coverage gates: 62 soft-fail scenarios, 121 strictness rows, 344 keyword rows, 171 classified BNF rows, and 249 classified Loc-node rows.
• Direct Loc fixture coverage: 152 covered rows, 97 deferred rows with approval metadata.
• BNF implementation debt is explicit: 11 covered, 86 partial, 47 deferred, 27 catalog rows.

Verification

• go test -count=1 ./oracle/...
• go test -run 'TestOracleParserProgress|TestOracleCoverage|TestOracleBNFImplementationDebtRequiresApproval|TestOracleKeywordOfficialSQLReserved26aiAudit|TestOracleLocNodeCoverage|TestOracleLocNodeFixtureCoverageRequiresApproval|TestVerifyCorpus|TestSoftFail|TestStrict' -count=1 ./oracle/parser
• ORACLE_PARSER_REF_CONTAINER=1 go test -tags oracle_ref -run 'TestOracleReference|TestOracleVReservedWordsKeywordAudit' -count=1 -v ./oracle/parser
• git diff --cached --check

Notes

• This PR makes Oracle parser gaps auditable and enforced by tests; it does not claim complete Oracle grammar coverage.
• The remaining parser scope is tracked as partial/deferred/catalog rows in the BNF and Loc manifests.