feat: add PHP mb_strwidth (AOT + Magician)

[nahime0]

Adds PHP mb_strwidth() on both AOT and Magician/eval(), matching PHP 8.5 East Asian Width for UTF-8 and the same encoding argument rules as mb_strlen().

Why

Termwind layout (Styles::getLength and related width math) needs mb_strwidth. Elephc mbstring previously exposed only mb_strlen and mb_ereg_match.

What

• Shared PHP 8.5 mbfl_eaw_table + character_width() in elephc-builtin-contract
• Catalog contract, AOT builtin! home, Magician eval_builtin! hook
• __rt_mb_strwidth / __rt_mb_char_width on every supported target, gated with the existing mb_strlen runtime family
• Encoding aliases (UTF-8/UTF8, 8bit/binary/7bit), iconv-backed names, catchable ValueError for unknown encodings
• Codegen, eval-parity, Magician, and error tests; examples/string-ops demo
• Generated builtin docs, docs/php/strings.md, runtime internals, and ROADMAP

Scope

Only mb_strwidth. No other mb_* functions.

Verification

Focused tests are green locally on linux-x86_64:

• cargo test --test codegen_tests test_mb_strwidth (4 tests)
• cargo test --test codegen_tests test_eval_mb_strwidth
• cargo test --test error_tests test_error_mb_strwidth (3 tests)
• Magician execute_program_dispatches_mb_strwidth_builtin
• Shared EAW table unit test
• function_exists('mb_strwidth') and first-class mb_strwidth(...) on AOT and eval
• Builtin docs pipeline (extract_builtins, audit, site compat, EIR boundary, gen_php_comparison)

CI covers the full supported-target matrix.

Focused test log
AOT and eval demo

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[greptile-apps]

Too many files changed for review (282 files, 100 file limit).

Bypass the limit by tagging @greptile-apps to review.

[nahime0]

Superseded by #952, which centralizes the mbstring implementation across the shared contract, AOT, Magician/eval, supported targets, tests, examples, and generated documentation. Closing this standalone PR in favor of #952.