applib/graphics: add a minimal BiDi engine

[kaluaim]

Following the discussion in #1631 (comment) — mirroring paired brackets on its own does not cover mixed-direction text, so this does the reordering properly.

Non-RTL text never reaches the engine

bidi_is_needed() gates the whole path on a raw byte scan over the same range the previous utf8_contains_rtl() covered. Text with none of those bytes costs one comparison per byte, never decodes UTF-8, and keeps taking the existing left-to-right path. No behaviour change for Latin-only text.

What it fixes

• Bracket mirroring (L4). RTL runs are reversed but the glyphs are never mirrored, so (مرحبا) draws as )مرحبا(. main has no mirroring at all.
• Neutral resolution (N1/N2). Spaces and punctuation join whichever run happens to be open instead of resolving against their neighbours, so the two halves of a bracket pair can land in different runs and be treated inconsistently.
• Combining marks (W1). A mark is reversed away from the letter it attaches to, and can resolve into a run of its own.
• European numbers (W7). A number after a Latin word is treated as right-to-left, so مرحبا abc 123 def renders the embedded phrase as def 123 abc.
• Paragraph separators (class B). Direction resolution runs past \n into the following paragraph.

What it preserves

The weak-LTR digit and numeric-separator handling already in rtl_support.c is kept, not added. It moves from a special case inside the reversal into the run classification: European and Arabic-Indic numbers resolve to their own left-to-right run, which is the level UAX 9 assigns them in either paragraph direction. ٢٠٢٦ and ١٢:٣٤ behave as they do today.

Resolving neutrals properly also removes the need to peel trailing spaces into a separate segment, so rtl_segment_content_end() goes away with it. Lam-alef ligature shaping and the segment cap are untouched.

Scope

Implemented: P2/P3 (paragraph direction), W1 (marks), W4–W7 (numbers keep their separators and terminators), N1/N2 (neutrals), L4 (mirroring, 10 pairs), class B (paragraph separators).

Not implemented: explicit directional controls (LRM/RLM/RLE/PDF), isolates, embedding levels above two, the full ~380-character mirror set. N0 is not implemented — N1/N2 already give both halves of a bracket pair the same direction in the cases that arise in practice.

Changes

rtl_support.c becomes bidi.c, rewritten. The run splitter in text_layout.c collapses into a single bidi_next_run() call, and mirroring is applied in both the width pass and the draw pass so the two agree.

About 2.1 KB of flash (.text 2012 B + .rodata 90 B, cortex-m4 -Os), no RAM, no allocation. It replaces rtl_support.c at 550 B and text_layout.c is net 74 lines smaller, so the firmware grows by roughly 1.3–1.5 KB.

Testing

test_rtl_support.c is replaced by tests/fw/test_bidi.c — 29 cases. The digit and separator coverage from the old file is kept, re-expressed as run boundaries rather than reversal output, alongside bracket mirroring in both paragraph directions, combining marks across every block that carries them, \n as a paragraph boundary, regional-indicator pairing, and the fast-path gate. All pass, clean under ASan and UBSan.