fix: scope every flag lookup to the region the tool actually parses

A review round found the same defect wearing four different shapes, so the
fixes are structural rather than per-site.

The tool's own args end at `--`. Msbuild keeps classifying past the boundary
because it forwards arguments, so every dotnet lookup read what the user meant
for the test runner: `dotnet test -- --logger trx` suppressed RTK's own logger
injection and `dotnet build -- -bl` its binlog. `before_dashdash` now scopes
the five dotnet lookup primitives at once, and `--report-trx` -- legitimate in
either region -- says so where it reads both.

Injected flags belong before that boundary too. `dotnet format -- ./src` put
`--verify-no-changes` after it, where dotnet parks it unread: format rewrote
the tree while RTK reported check mode. `git diff --stat` had the same shape.
Both go through `injection_point` now.

Detection and the action on it have to share one rule. `--no-compact` was
detected as a token and stripped as a string, deleting a pathspec of that name;
`--write` was detected bare-and-unforwarded but stripped everywhere. They now
strip the tokens detection actually saw.

`stash` broke restore_double_dash's precondition: clap carves its subcommand
out of the same trailing region, so restoring from the remainder sliced one
token short -- `git stash -- -p` ran an interactive `push -p` where real git
errors on the pathspec. The region is reassembled before restoring.

Also from the round, each verified against the real tool: `-f`/`--file`
supplies patterns like `-e`, so `grep -f pats.txt a.txt` no longer answers "no
matches" (or hangs) by stealing the path as the pattern; grep's `-NUM` is
`--context=NUM`, so its blocks keep their `--` separators; a single matching
file found by walking the cwd keeps its name, as real rg prints it; `git diff
-p` stopped printing the patch raw and then again compacted, 2.4x the raw
output; `/Other.csproj` is a path, not an MSBuild switch, so it no longer
adopts an unrelated project's runner mode; and `/l:trx` is MSBuild's logger
assembly, not dotnet's `--logger`, which the tokenizer now records as
`Token::slash`.

git's remaining string-scanning handlers move to the tokenizer with the rest:
`git status --` keeps its compact path, a blob spec is a free positional rather
than any arg containing a colon, and a worktree action is the subcommand rather
than any arg spelling one.

The two stdin regressions move to tests/ where CI actually runs them -- they
were `#[ignore]`d, and CI never passes `--ignored` -- and the show_line/context
tests now drive the production detector instead of test-only twins of it, which
let a deleted production arm stay green. The twins are gone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>