Fix correctness bugs and reduce hot-path allocations found in code review

[AlternativeLua]

Summary

A code review pass over the compiler surfaced a batch of correctness bugs and hot-path performance issues. This PR lands them in two commits: one fix: commit for behavior bugs and one perf: commit for allocation/overhead reductions.

Correctness (fix: commit)

• Location.Equals compared only file + column, so positions on different lines with the same column were "equal", and GetHashCode disagreed with Equals. Now both use file + position.
• TypeChecker.VisitIf read the else branch's exit state before visiting it, so a terminating else branch (else { return }) was invisible to definite-assignment merging.
• Generated Luau strings didn't escape \ or the quote delimiter, producing broken output; long-bracket strings also lost a leading newline (Luau swallows the first one after [[).
• RenderState.PopIndent hardcoded 2 characters, breaking any non-default Indent.
• The parser silently dropped mut in expression position when not followed by an array literal; it now reports a diagnostic (for x : mut arr still parses).
• TypeSolver.CheckCircular lost its ref writes through a lambda copy, so circular union/intersection members were never replaced with never.
• The lexer's unterminated-string-at-newline check was dead code — the scan loop never stopped at \n. Strings now terminate at newlines and the existing diagnostic fires (matches the existing LexerTest expectation).
• NodeId.Map retained every AST node ever constructed in an unsynchronized static dictionary. Removed; MustImportRuntimeLibrary records what it needs at reference time in the resolver instead.
• VisitAssignmentOperator could throw NRE/InvalidCastException on malformed nested assignments instead of leaving a diagnostic.
• Orphaned <=/>= comparisons were emitted as bare expression statements (invalid Luau) because the assignment check only excluded ==/~=.
• Fail-fast exit code was a randomized string hash (nondeterministic per process); now a stable 1.
• Parser.Expect/PeekKind could index out of range at position 0 / past the token list.

Performance (perf: commit)

• BindType ran TypeSimplifier.Simplify, formatted a message, and hashed a debug diagnostic (forcing line/character resolution) for every node bind. Now gated behind TypeChecker.EmitDebugDiagnostics (default off) — this restores the benefit of the recent TextSpan work for the type-checking stage.
• Type.IsNever/IsUnknown are pattern matches instead of Equals calls through the static visited-set guard (two HashSet operations per call).
• The empty MacroContext is cached and invocation-macro classification runs once per visit instead of twice per identifier/property access.
• Node construction uses plain loops + List.Sort instead of LINQ Where/Cast/OrderBy/ToList chains.
• Diagnostic.SourceLines is cached instead of re-splitting the whole file per access; DiagnosticBag.ContainsErrors no longer allocates intermediate bags.
• Removed dead lexer code left over from the operator-trie migration.

Test plan

• Full suite passes: 1701/1701 (up from 1695 — six new tests cover the Location equality contract, the VisitIf exit-state fix, string escaping, and the orphaned-mut diagnostic).
• The json_stringify Luau snapshot was updated for the now-correct \\n escaping.
• Each commit builds independently.

🤖 Generated with Claude Code