WidthBudgetRule class

Warn when a string is too long for the UI slot it renders in.

This is the "size-aware translation" guard. Text expands when translated — "Edit profile" (12) → "Chỉnh sửa trang cá nhân" (23) — and a faithful-but-long value silently breaks a tight button. The LengthRatioRule can't catch that: 23/12 is 1.9×, well inside its [0.3, 2.5] smell-band. What's missing is a budget tied to the slot.

Opt-in by construction. A key is only checked if its SOURCE @key block declares a budget. Body copy, legal text, empty-state prose — anything with room — is never annotated and never policed.

Two ways to declare a budget on the source @key:

"editProfile": "Edit profile",
"@editProfile": { "x-slot": "button" }      // policy from dialect.yaml

"statusChip": "Live",
"@statusChip": { "x-max-length": 6 }         // hard character cap

Slot policies live once in dialect.yaml:

slots:
  button: { max_ratio: 1.4 }   # stay within ~1.4× the source ("similar length")
  chip:   { max_length: 10 }   # a real pixel slot → absolute cap
  tab:    { max_ratio: 1.2, grace: 2 }

A max_ratio budget is max(round(sourceLen × ratio), sourceLen + grace) — the grace floor (default _defaultGrace) keeps the shortest labels ("Save" → "Lưu") from tripping on a ratio that's meaningless at 3 chars. max_length is absolute and can flag an over-tight source (English itself busting the slot) as well as a translation.

Severity is warning, and --strict alone does NOT promote it — like length_ratio, it needs the explicit --strict-length opt-in. A button that runs a few chars long is a nudge for the next author/agent to tighten, not a reason to fail the build. It's a convention, enforced softly (and ack-able), by design.

Length is measured on the literal text (IcuMessage.literalText) in Unicode code points, so ICU placeholder names don't inflate the count. A value with runtime placeholders is inherently fuzzy to budget; put budgets on tight static labels, which is where overflow actually bites.

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width_budget library