fix(bash): preserve whitespace after empty assignments

[denisvmedia]

Fixes #1106.

! A=x B= command must leave B empty and preserve command as the command name. The opening-parenthesis probe consumed the separating whitespace before the scanner reached EMPTY_VALUE. Some inputs produced a missing word. Others produced an incorrect assignment without an error flag.

Preserve whitespace when EMPTY_VALUE is valid. The existing scanner then emits its zero-width token. The change adds one condition and leaves grammar data, token identities, and parser recovery unchanged.

Add shared inputs and blackbox tests:

• Check assignment values, command names, byte ranges, and HasErrorOrMissing().
• Cover negation, multiple assignments, tabs, CRLF, Unicode, substitutions, redirects, pipelines, case bodies, arrays, and quoted values.
• Keep incomplete syntax invalid.
• Compare valid inputs with the locked C grammar across strict, production, and raw Go parsing.
• Include the new C comparison in the existing CI job.

Validation used a dedicated Linux Docker container with two CPUs and an 8 GiB memory limit. No repository-wide host sweep ran.

• Before the fix: 12 of 18 new cases failed.

• With the fix: all 18 cases and available Bash regressions passed.

• The locked C comparison passed for 16 valid inputs across three routes. It checks symbols, fields, byte/point spans, flags, child order, and deep digests.

• Existing Bash fresh/incremental parity and C-reference regressions passed.

• The final fix passed Bash race tests on Go 1.27.0.

• Bash tests passed with CGO_ENABLED=0 on Go 1.22.0, the module's declared minimum compiler.

• Actionlint passed for the changed workflow.

TestBashForestRepetitionFoldDispatch skipped because its optional real-corpus fixture was absent.

Reproduce the scoped checks inside a Docker container:

go test -race ./grammars -run '^Test(Issue1106Bash|Bash)' -count=1
CGO_ENABLED=0 GOTOOLCHAIN=go1.22.0 go test ./grammars -run '^Test(Issue1106Bash|Bash)' -count=1
cd cgo_harness
go test . -tags treesitter_c_parity -run '^(TestBash.*COracleParity|TestIssue1106BashEmptyAssignmentsCParity)$' -count=1
GTS_PARITY_MODE=smoke go test . -tags treesitter_c_parity -run '^TestParity(FreshParse|IncrementalParse|HasNoErrors)$/^bash$' -count=1

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