Rename exit→fin, remove continue/return aliases, fix BMark parity

[Copilot]

Completes the exit→fin keyword migration, removes backward-compat lexer aliases for continue and return, and fixes a parity test failure in BMark.cod caused by un-normalized timing output.

Keyword rename: exit → fin

• TokenType.Keyword.EXIT → FIN; Exit AST node → Fin; EarlyExitException → EarlyFinException
• IR serialization tag updated: "Exit" → "Fin" (no backward compat needed; pre-release)
• All parser, lowerer, visitor, and printer references updated

Alias removal (IdentifierLexer)

• Dropped continue → SKIP and return → FIN fallback aliases — only explicit skip / fin are valid control-flow keywords now

.cod source updates

All stdlib and demo files migrated to the new keywords:

// before
if done { return }
for i of 1 to n {
    if i == skip_val { continue }
    ...
}
exit

// after
if done { fin }
for i of 1 to n {
    if i == skip_val { skip }
    ...
}
fin

Files changed: std/json/Json.cod, std/scimath/SciMath.cod, demo/.../ControlFlow.cod, demo/.../JsonStandardLibraryComprehensive.cod

Parity fix

CodPTACParityRunner.normalize() now redacts timing values in "... at <N> ms" output lines, resolving the BMark.cod AST-vs-PTAC mismatch caused by non-deterministic timer() output.