fix: single-quote literalness (B2) + LooksLikePath trailing-backslash (B3)

[Aaronontheweb]

Summary

Two parser correctness fixes pulled from the v0.1.x backlog. Public API surface unchanged.

B2 — single-quote literalness

v0.1.0-alpha resolved $HOME uniformly regardless of quote style, so echo '$HOME' produced Kind=Tilde. Per SPEC §5, single-quoted contents are literal bytes — no variable expansion. The lexer now marks single-quoted QuotedString tokens with an internal IsSingleQuoted flag, and the resolver short-circuits tilde / $HOME / $VAR / glob / filesystem:: handling when set. Matches bash semantics.

• echo '$HOME' → Kind=Literal, Resolved=null (was Kind=Tilde)
• cat '/etc/passwd' → still resolves to /etc/passwd (literal absolute path inside single quotes)
• rm '$VAR/file.txt' → Kind=Literal, Resolved="/work/$VAR/file.txt" (literal under cwd)

B3 — LooksLikePath trailing-backslash carve-out

A double-quoted token like "foo\\" lexes to Value foo\ (escape collapse). The trailing \ is an artifact, not a path signal, but the heuristic was returning IsPath=true. Tightened: forward slash anywhere still counts; backslash counts only at a non-trailing position.

• echo "trailing backslash\\" → IsPath=false (was IsPath=true, Resolved=/work/trailing backslash)
• cd dir/ → still IsPath=true, Resolved=/work/dir (regression guard)
• path\to\file → still IsPath=true (non-trailing backslash)

What changed

File	Change
src/ShellSyntaxTree/Internal/Bash/Lexing/BashToken.cs	New IsSingleQuoted init-only flag on the internal record (additive).
src/ShellSyntaxTree/Internal/Bash/Lexing/BashLexer.cs	ReadSingleQuoted sets IsSingleQuoted=true on its emitted token.
src/ShellSyntaxTree/Internal/Resolving/BashResolver.cs	New Resolve overload accepting isLiteralBytes; LooksLikePath updated for backslash carve-out.
src/ShellSyntaxTree/Internal/Bash/Parsing/BashCommandParser.cs	Threads token.IsSingleQuoted through to the resolver from the QuotedString case.
SPEC.md §8	New Step 0 (Single-quoted bypass) preamble + LooksLikePath heuristic clarification.
tests/.../Corpus/bash/104, 109	Updated to assert corrected outputs.
tests/.../Corpus/bash/119, 120, 121	New regression-guard entries.
Directory.Build.props + RELEASE_NOTES.md	Bump to 0.1.2-alpha.

Public API impact

Zero. No new public types/fields, no signature changes. PublicApiSnapshotTests count unchanged.

Verification

• pwsh ./scripts/Add-FileHeaders.ps1 -Verify — clean.
• dotnet build -c Release — 0 warnings, 0 errors.
• dotnet test -c Release — 359 / 359 passing (was 356; +3 new corpus entries).
• dotnet pack -c Release -o ./bin/nuget — ShellSyntaxTree.0.1.2-alpha.nupkg + .snupkg produced cleanly.

Test plan

• Corpus entries that previously pinned the bad behavior (104, 109) now assert the fixed outputs.
• Regression guards for the correct behaviors that should NOT have shifted (119 single-quoted absolute path resolves, 120 cd trailing-slash still IsPath, 121 single-quoted env-var stays literal).
• CI green on ubuntu-latest + windows-latest.

What follows this PR

After merge: push 0.1.2-alpha tag to fire trusted-publishing → nuget.org.