t: add lint-style.pl and convert grep to test_grep

[mmontalbo]

The test suite has a test_grep wrapper that prints file contents on
assertion failure, making debugging easier. Many tests still use
bare 'grep' for assertions, which silently swallows context on
failure.

This series adds a lint tool (lint-style.pl) to mechanically detect
and convert these, then applies it across the test suite.

The tool reuses the Lexer and ShellParser from chainlint's shared
parser module rather than reimplementing shell parsing. This
gives us proper handling of heredocs, $(...), pipes, quoting,
case patterns, and control flow structures for free. The only
new parsing logic is the grep assertion classifier, which works
on the already-parsed token stream.

The approach:

• Positive matching: is_grep_assertion() examines parsed tokens
  and classifies whether a grep looks like an assertion (PATTERN +
  FILE) vs a filter (piped, redirected, inside $(), with -c/-l/-L).
  Only assertions are converted.

• Missing file detection: greps at assertion level with no file
  argument (reading from empty stdin) are flagged as likely bugs.
  Three pre-existing instances were found and fixed in patch 5.

• Auto-fix: --fix applies the conversions mechanically. The -q
  flag is dropped (test_grep checks status natively), and
  negation is rewritten from '! grep' to 'test_grep !'.

Structure:

1/6 t/README: document test_grep helper
2/6 t: extract chainlint's parser into shared module
3/6 t: fix Lexer line count for $() inside double-quoted strings
4/6 t: add lint-style.pl with test_grep negation rule
5/6 t: fix grep assertions missing file arguments
6/6 t: lint and convert grep assertions to test_grep

Patch 2 extracts chainlint.pl's Lexer, ShellParser, and
ScriptParser into a shared module (t/chainlint/lib-perl.pl).
ScriptParser's check_test() becomes a no-op in the module;
chainlint.pl defines ChainlintParser (a ScriptParser subclass
that runs TestParser for &&-chain detection), and lint-style.pl
defines LintParser (a ScriptParser subclass that runs grep
lint rules). chainlint.pl loads the module via do() and
behaves identically.

Patch 3 fixes a pre-existing Lexer bug where multi-line $()
substitutions inside double-quoted strings caused line numbers to
drift. chainlint is unaffected (it uses byte offsets), but
lint-style.pl needs accurate token line numbers to locate and fix
specific lines in the original file.

Patch 4 introduces the lint-style.pl framework (LintParser
subclass, Makefile targets, fixture infrastructure) with a small,
complete rule (! test_grep -> test_grep !) so reviewers can see
the machinery in action before the bulk conversion.

Patch 5 fixes three test bugs where grep assertions were missing
their file arguments, causing them to pass vacuously (all three
pass with the corrected arguments). These were independently
discovered by the missing-file detection rule introduced in
patch 6.

Patch 6 adds the main rule, its fixtures, and the mechanical
conversion of ~2800 assertions across ~340 files, including sourced
test fragments in t/t5411/ and heredoc test bodies in t/t5564/.

To verify the conversion (patch 6 adds both the rule and the
mechanical conversion in the same commit, so apply the full
series and re-run --fix to confirm it produces no further
changes):

git checkout &&
perl t/lint-style.pl --fix t/t*.sh t/test-lib*.sh t/lib-.sh
t/-tests.sh t/perf/.sh t/t5411/.sh

As an independent completeness check:

grep -rn '^\s*!\sgrep\s' t/t*.sh t/lib-.sh t/-tests.sh
t/test-lib*.sh t/perf/*.sh |
grep -v test_grep | grep -v lint-ok | grep -v '#.*grep'

Known limitations:

• Two greps in t/t3901 inside case branches that inherit piped
  stdin from two lines above are suppressed with '# lint-ok'.

• One grep in t/t6437 uses glob expansion (grep -q content *)
  which breaks test_grep's file check; suppressed with '# lint-ok'.

• One grep in t/t7527 captures $? for later use rather than
  asserting inline; suppressed with '# lint-ok'.