resources/lang: add th_TH Thai locale (fonts)

[ninefyi]

Summary

Adds Thai (th_TH) as a new locale, following the pattern of the existing
ar_SA / he_IL / zh_CN locales. Font resources only — no firmware or
tooling changes.

• thai_codepoints.json — the 87 assigned Thai Unicode codepoints
  (U+0E01–U+0E3A, U+0E3F–U+0E5B). Thai-only, mirroring arabic_codepoints.json:
  mixed-script text falls back to the base Latin font via the existing depth-1
  fallback in text_resources.c.
• lang_map.json — GOTHIC_14/18/24/28/36 (+bold) EXTENDED entries at the same
  pixel heights ar_SA uses. BITHAM/ROBOTO/DROID_SERIF left unchanged.
• Sarabun-Regular.ttf / Sarabun-Bold.ttf + LICENSE (SIL OFL 1.1). Static
  per-weight files are required because Noto Sans Thai now ships only as a
  variable font, which this pipeline cannot consume.

Translations are intentionally not included

Per CONTRIBUTING.md, translated text is not submitted through pull requests.
lang_map.json therefore sets strings.file: "" and ships fonts only, as ar_SA
does. Thai strings should flow through Crowdin once th_TH is enabled there.

This does not prevent Thai from being useful: notification content arrives from the
phone and is rendered by these fonts, independent of the UI string catalog. Verified
by installing a fonts-only pack (strings.file: "") and sending a Thai notification —
it renders and wraps correctly. Only system UI labels remain English until
translations land.

Validation

• ./pbl pack_lang --lang=th_TH builds all 10 font variants across the full
  87-codepoint set with zero glyph errors.
• Glyph budget checked against both MAX_FONT_GLYPH_SIZE limits: fits within
  256 B (flint/diorite/chalk/basalt/aplite) as well as 512 B (emery/gabbro).
• Rendered live in QEMU on all three shipping platforms:
  • qemu_emery (Pebble Time 2)
  • qemu_flint (Pebble 2 Duo — 256 B glyph limit, 1-bit display)
  • qemu_gabbro (Pebble Round 2 — round display / perimeter text path)
• Word wrapping verified through the real notification path. Thai has no spaces, and
  without U+200B the layout engine breaks mid-syllable and hyphenates; with U+200B at
  word boundaries it breaks cleanly. No overflow or clipping in either case.
• Mixed Thai/Latin (sender names, URLs, 15:30) renders correctly with no wildcard
  glyphs.
• Thai glyph coverage, readability, mixed Thai/Latin rendering, and stability
  confirmed on physical Pebble Time 2.

Known limitation

On physical PT2, the sequences กี่ and สี่เหลี่ยม show tone-mark overlap. This
renderer applies no OpenType mark positioning — glyphs are placed purely by the
font's own advance metrics — so mark placement depends entirely on the font. Most
sequences tested (including tall-ascender cases such as ปิดเครื่อง and
เปิดใช้ด่วน) render cleanly; these two do not.

This PR does not claim complete Thai mark positioning for every combining sequence.
Addressing it properly means either a different font or mark-positioning support in
the renderer, which is out of scope here and better discussed separately.

Contributor checks

• Sarabun license and attribution reviewed.
• Generated code manually reviewed.
• DCO sign-off present.
• AI assistance disclosed.

[ninefyi]

Context and open questions for this PR are in #2046 — in particular whether th_TH can be enabled on Crowdin (this PR is fonts-only and carries no tintin.po), and how you'd prefer the combining-mark overlap on กี่ / สี่เหลี่ยม to be handled.

[gmarull]

@jplexer pls check since you've been changing languages stuff recently