Add printf alphabet matrix (mix bash_fixtures.gen printf)

[davydog187]

Implements #70 item 2 for printf only: a generated printf_matrix suite in the same generate / record / test path as date_matrix.

The old by-example printf.json (6 cases) is unchanged. Oils builtin-printf.test.sh is a cross-check, not this matrix.

What was enumerated

mix bash_fixtures.gen printf writes 595 cases:

• Every conversion bash/coreutils documents: %s %c %d %i %u %o %x %X %f %e %E %g %G %a %A %%, plus bash extras %b and %q
• Each conversion at two or more bases so padding, signs, empty and zero are visible (hi vs empty, 7 vs 255, -1 vs 0)
• Flags (-, +, space, 0, #) crossed with every conversion, and again with width 5
• Widths 1/5/10, precisions 0/2/6 and empty %.d, plus %5.2
• Finite * forms (%*, %.*, %*.*, flag+*) — extra arguments are the width/precision, not the value
• %b inputs that make escapes visible (\t, \n, \x41, \101, \c)
• %q inputs that make shell-quoting visible (empty, space, quote, $HOME, *, -n)
• Format recycling with excess arguments (printf '%s' a b c, mixed %s:%d, %b/%q recycle)
• Command-level cells: no operands, --, -v, -Z, %(fmt)T

mix bash_fixtures.gen printf --dry-run reports printf_matrix: 595 cases. mix bash_fixtures.gen now generates both date and printf.

Recording

Docker was not available on the agent VM. Oracle output was recorded with the same test/fixtures/runner.sh the Docker task runs, under GNU bash 5.2.21 / Ubuntu 24.04, then pretty-printed through Mix.Tasks.BashFixtures.write_json!/2:

mix bash_fixtures.gen printf
bash test/fixtures/runner.sh < test/fixtures/bash_cases/printf_matrix.json > /tmp/printf_matrix_raw.json
# then Jason-pretty-print + Fixtures.validate, as `mix bash_fixtures` does

To re-record through Docker when it is available:

mix bash_fixtures printf_matrix

\xff is not in the matrix: jq --rawfile replaces invalid UTF-8, so that cell would be a harness artifact. The existing unit test still covers it.

Known gaps (467 of 595)

No cell was omitted. Divergences are opts.known_gap (excluded from the digest). The fixture runner still executes them with the assertion inverted.

Count	Reason
187	Unimplemented conversions (%i %u %E %g %G %a %A %q) passed through literally
144	* for width/precision not implemented
54	+, space, # format flags not implemented
22	%e uses Erlang ~e (exponent width / default digits differ)
18	bash rejects a flag/width/precision on %%
8	empty precision (%.d) is not parsed
6	integer precision ignored (bash: minimum digit count)
4	invalid/incomplete specifier: JustBash exits 0, bash diagnoses
4	integer operands: hex 0xff, octal 010, \'A, leading spaces
3	%o/%x/%X of -1 printed signed; bash unsigned-wraps 64-bit
3	0-padding honoured on %s/%c/%b; bash ignores 0 there
3	precision on %b ignored
2	%c of empty/missing emits nothing; bash emits NUL
2	%(fmt)T not implemented
1	%b \NNN octal (only \0NNN, matching echo -e)
1	%b \c terminator
1	%0-5d flag order not parsed
1	no operands: exit 0 vs usage/exit 2
1	-- treated as the format
1	-v treated as the format
1	-Z absorbed as the format, exit 0

JustBash.exec/2 did not raise on any cell. No printf implementation bugs were fixed in this PR — it is the enumeration + harness. Cheap follow-ups the matrix now makes un-skippable: %i as a %d alias, unsigned wrap, +/ /# flags, *, %q.

Tests

All local quality gates passed:

• mix compile --warnings-as-errors
• mix format --check-formatted
• mix credo --strict (only the existing intentional banned_fixture_apply finding)
• mix dialyzer (zero new errors)
• mix test — 6094 tests, 0 failures
• mix test --only suite:printf_matrix — 595 tests, 0 failures

Related Issues

Related to #70 (item 2 only; not closing the issue)

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• Added new tests
• All existing tests pass
• Tested manually (mix bash_fixtures.gen printf --dry-run, local runner.sh recording, JustBash vs oracle classification)

Checklist

• My code follows the style guidelines of this project
• I have run mix format
• I have run mix credo and addressed any issues
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• I have updated the documentation accordingly
• I have updated the CHANGELOG.md (for non-trivial changes)