Implement immutable F# syntax trees and parser recovery

Parent: #8
Blocked by: #28

Outcome

Replace the hand-picked prototype parser with immutable syntax trees and recoverable parsing for F# implementation and signature files.

Acceptance criteria

• The syntax model covers namespaces, modules, opens, attributes, signatures, declarations, type syntax, patterns, expressions, members, accessibility, and ordered multi-file inputs.
• Implementation and signature files retain exact ranges and enough structure for later resolution and diagnostic recovery.
• Layout-sensitive and ambiguous forms follow declared language-version behavior.
• Parser recovery reports stable diagnostics while preserving subsequent independently parseable declarations.
• Syntax-only tests exercise generated SDK inputs and compiler-owned feature programs without requiring semantic phases.
• Adding a production rule is justified by an F# construct and conformance row, never by a named line or declaration in an external corpus.

Decision inputs

Issues #2 and #6; ADRs 0008, 0012, and 0014.

This issue inherits the corpus-neutral acceptance policy in #8.

Start by reading parent issue #8, blocker #28, issues #2 and #6, and ADRs 0008, 0012, and 0014 to understand the parser and corpus-neutral acceptance policy. Done means immutable syntax trees and recoverable parsing cover the listed F# constructs for implementation and signature files, preserve ranges, produce stable diagnostics, and pass syntax-only tests for the specified inputs.