[tse-wei-chen]

Scope

#245 advances the six-provider capability-expansion phase after #241–#244. The focus is JSON, regex, and provider-native advanced expression spellings while keeping source spelling -> typed semantic -> capability/profile proof -> target lowering -> native spelling.

Completed

Regex source grammar

• PostgreSQL ~ and !~ parse directly into the closed F# RegexMatch semantic.
• MySQL REGEXP, RLIKE, NOT REGEXP, and NOT RLIKE normalize to the same semantic.
• Wrong-dialect spellings fail at the source-dialect boundary.
• The CLR compatibility seam now carries a dedicated RegexExpr instead of round-tripping through the reserved CORE_REGEX_MATCH function-name string.

Algebraic JSON path

• Added typed JsonPathSegment = JsonProperty | JsonArrayIndex and private JsonPath.
• JSON validation and rendering share the same algebraic path parser instead of duplicated path regex interpretation.
• Constant non-negative array-index extraction is enabled for PostgreSQL/MySQL/SQLite where the existing JSON extraction contract is declared.
• Array-index mutation remains fail-closed because missing/out-of-range mutation semantics are not proven equivalent across providers.

MySQL JSON arrow

• MySQL -> is source-version gated at 5.7.9+.
• The proven source subset requires a JSON column identifier on the left and a literal JSON path on the right.
• It canonicalizes to the existing JSON extraction semantic.
• MySQL ->> remains fail-closed because its JSON_UNQUOTE(JSON_EXTRACT(...)) text-result semantic is distinct.

PostgreSQL JSON arrow

• Added a dedicated closed F# PostgresJsonAccess node rather than lowering PostgreSQL -> into generic CORE_JSON_EXTRACT.
• Selector shape is typed as property key vs signed 32-bit array index.
• Result shape is typed as JSON vs text, preserving the semantic difference between -> and ->>.
• The node survives Parse -> CLR compatibility AST -> F# compiler round-trips structurally through PostgresJsonAccessExpr.
• Rendering stays native PostgreSQL so json is not silently coerced to jsonb.
• PostgreSQL JSON arrow is accepted for 9.3+ semantics; an explicitly older source/target profile fails closed.
• Cross-provider lowering remains fail-closed until JSON/jsonb/text type equivalence is proven.

Validation

• Grammar bombardment floor: 6,088 total cases.
• Latest Tests workflow: #1419 — success (11/11 jobs).
• Latest CodeQL Advanced workflow: #1522 — success.
• Differential parity job is green.
• Package smoke is green.
• No unresolved review threads or review comments.

Safety boundaries retained

• No arbitrary JSON path text is admitted as executable SQL.
• Unsupported/dynamic JSON path shapes remain fail-closed.
• SQLite/Firebird regex remains rejected without an explicit native contract.
• PostgreSQL JSON arrow is not cross-lowered by guessing result-type equivalence.
• Provider/version-sensitive source grammar requires the declared capability proof where necessary.
• Existing main capability coverage remains a floor; this PR only adds supported grammar/capabilities.