editor: smart unit suffix — dim on focus, yield to explicit units

[Aymericr]

What does this PR do?

Reduces confusion in unit-typed inputs now that Lingo accepts explicit units (5 ft in a meters field). Previously the static m suffix stayed fully visible next to such a draft, reading as "5 ft m". Now, in both SliderControl and MetricControl:

• Editing, bare number/empty: the suffix dims to 40% opacity — it signals the default unit a bare number commits as.
• Editing, explicit unit typed: the suffix fades out (150ms opacity, no layout jump) and the existing = 1.52 m Lingo hint is the unit feedback.
• Unparseable draft: suffix stays dimmed (the field reverts on commit).
• At rest: unchanged.

The show/hide predicate is hint !== null, so it can never disagree with measurementHint's plain-number guard.

How to test

1. bun dev, select a wall and open the inspector.
2. Click a length value: suffix dims. Type 5 — suffix stays dimmed. Enter commits 5 m.
3. Type 5 ft — suffix fades out as soon as the draft isn't a plain number; the = 1.52 m hint appears. Enter commits 1.52 m and the normal suffix returns.
4. Type garbage (abc) — suffix stays dimmed; Enter reverts the field.
5. Repeat in a stair/elevator panel (MetricControl) and on a %/unitless field (dim only, never hides).
6. Toggle imperial and verify the same behavior with the ft suffix.

Screenshots / screen recording

N/A — will add a clip if requested (subtle opacity-only change).

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

🤖 Generated with Claude Code

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Note

Low Risk
Inspector-only opacity styling on unit suffixes during text edit; no parsing, commit, or stored-value logic changes.

Overview
MetricControl and SliderControl now treat the inline displayUnit suffix differently while the value field is focused, so typed measurements like 5 ft in a meters field no longer read as “5 ft m”.

During edit mode, a bare number or empty draft keeps the suffix visible but dimmed (text-muted-foreground/40) to signal the default unit a plain number commits as. When measurementHint is active (explicit unit / non–plain-number draft that parses), the suffix fades out over 150ms (opacity-0 when hint is set), leaving the = … m hint as the unit feedback. At-rest display is unchanged.

The hide/show rule matches hint !== null, aligned with measurementHint’s plain-number guard.

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