diagram: sim-specs fields silently revert invalid input with no feedback

[bpowers]

Problem

In src/diagram/ModelPropertiesDrawer.tsx, SimSpecDraftField commits a draft on blur/Enter via resolveSimSpecDraft (src/diagram/sim-spec-draft.ts). When the draft is invalid -- e.g. a non-positive dt, or an empty/non-numeric start/stop time -- the resolver returns shouldCommit: false and the field silently discards the draft, snapping back to the model value on blur.

There is:

• no inline error text explaining why the typed value vanished,
• no aria-invalid / aria-describedby announcement for assistive tech,
• no visual error state at all.

To the user (sighted or not), a typed value simply disappears.

Why it matters

• Accessibility: screen-reader users get zero signal that their input was rejected; the field just reads back the old value.
• UX: silent reversion is one of the most confusing failure modes in a form -- the user has no way to learn what input the field would accept.

Component

src/diagram -- ModelPropertiesDrawer.tsx (SimSpecDraftField), sim-spec-draft.ts (resolveSimSpecDraft), components/TextField.tsx.

Possible approach

The shared TextField component already wires aria-invalid + aria-describedby to its helperText automatically (added 2026-07-13 during the mobile/accessibility pass on the production-risk-burndown branch), so the fix is mostly plumbing: have SimSpecDraftField derive a validation message from the draft state (e.g. "dt must be a positive number") and pass error/helperText to TextField instead of silently reverting. Options for commit semantics:

1. Keep the revert-on-blur behavior but show the error while the draft is invalid (message clears on revert or valid input), or
2. Hold the invalid draft in the field with a persistent error message until the user fixes it or presses Escape.

Either way resolveSimSpecDraft likely wants to return a reason alongside shouldCommit: false so the message can be specific per field/failure.

Related but distinct

• diagram: applySimSpecChange full-payload read-modify-write can silently revert a concurrent sim-specs commit (lost update) #833 tracks a concurrency lost-update in applySimSpecChange (a valid commit silently reverted by a stale concurrent payload). This issue is about invalid input being discarded by design with no feedback -- different root cause, different fix.

Context

Identified during the mobile/accessibility fix pass on the production-risk-burndown branch; deemed out of scope there because it changes commit/validation UX behavior rather than just wiring up existing error text.