Implement F# source text, lexing, directives, and layout

Parent: #8
Blocked by: #25, #26, and #27

Outcome

Implement the source-text and lexical layer for batch F# implementation and signature inputs.

Acceptance criteria

• The lexer handles Unicode identifiers, operators, keywords, numeric, character, byte, and string literal forms, comments, trivia, escaped identifiers, and compiler directives.
• Conditional compilation, line directives, source encoding, BOM handling, CRLF/LF differences, and indentation/offside behavior preserve exact source ranges.
• Language-version gates are data-driven and covered by the conformance manifest.
• Malformed tokens and directives produce recoverable, ordered Compatibility Oracle diagnostics.
• Token and trivia data are immutable and do not contain project-, file-name-, or external-corpus special cases.
• Cold and reused lexing return observably equivalent phase results.

Decision inputs

Issue #2; ADRs 0008, 0014, and 0018.

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

Start with the parent issue #8 and unblockers #25, #26, and #27, then read issue #2 and ADRs 0008, 0014, and 0018. Done means implementing the source-text and lexical layer while meeting the listed acceptance criteria for syntax coverage, directives, layout, diagnostics, immutability, and equivalent cold and reused results.