analyse: surface the 3D-stage → 2D fallback instead of failing silently

[DheerajNalapat]

What

The /analyse room view has three modes — stage3d, room2d, fixtures. When you pick 3D stage it can silently draw the 2D room instead, and stay stuck there: the toggle keeps saying "3D stage" while the flat 2D elevation renders, with nothing on the console or the screen to explain why. Only a page reload recovers it.

Root cause

ensureStage() builds the WebGL stage lazily. On any failure it did two harmful things:

catch(e){ STAGE3D=null; }        // swallowed the exception — no console, no UI
if(!STAGE3D) STAGE3D_OK=false;   // latched false for the whole tab — never retries

So a single failure to create the stage latched the tab into the 2D fallback permanently. The most common trigger on a machine where WebGL otherwise works: Chrome caps live WebGL contexts (~16), and several open Limelight tabs (/stage, club3d.html, multiple /analyse) exhaust them, so new THREE.WebGLRenderer() throws "Too many active WebGL contexts" — swallowed, latched, stuck.

This is exactly the "Silent failures cost more than ugly ones — surface the error" rule in AGENTS.md. Not a regression: the 3D path is correct and renders fine in a clean tab (verified — Intel Mesa WebGL, createStage3D returns a real stage). The bug is the silent, permanent degrade.

Fix (synth/analyse.html only)

1. Don't swallow — console.warn the real exception, and record the reason.
2. Surface it — a note beside the view button: "· 3D stage unavailable (<reason>) — showing 2D room", cleared when 3D is live.
3. Stop latching permanently — the one-attempt guard is now per selection (STAGE3D_TRIED), and re-selecting the 3D stage clears it, so a failure that has since cleared (a freed WebGL context) recovers without a reload.

Verification

Reproduced and verified on the running portal by forcing createStage3D to throw:

	before	after
console on failure	silent (0 warnings)	console.warn with the real error
on-screen notice	none	"3D stage unavailable (…) — showing 2D room"
re-select 3D after WebGL frees	still 2D (latched)	recovers to live 3D
healthy path	3D	3D, note stays empty

No other files touched.

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