fix(terminal): reduce WebGL context pool to leave headroom for Chromium limit

[gregpriday]

Summary

• Reduces TerminalWebGLManager.MAX_CONTEXTS from 16 to 12, leaving 4 slots free for any non-terminal WebGL consumers sharing the renderer process
• Chromium enforces a hard limit of 16 active WebGL contexts per renderer process; claiming the full budget means the first external consumer triggers a forced webglcontextlost eviction on a terminal
• Adds an inline comment documenting the reasoning and noting that browser/dev-preview panels run in <webview> partitions with their own budgets (so the reservation is conservative headroom for future renderer-resident consumers)

Resolves #3277

Changes

• src/services/terminal/TerminalWebGLManager.ts: MAX_CONTEXTS 16 → 12, with explanatory comment

Testing

All existing WebGL lifecycle unit tests pass. The change is a single constant reduction with no behavioral impact on the eviction/fallback path.

[gregpriday]

Review status: Ready

[gregpriday]

Cross-PR review note

After reviewing all 8 open PRs together, these interactions were identified:

• PRs fix(terminal): reduce WebGL context pool to leave headroom for Chromium limit #3358 and fix(terminal): skip WebGL renderer on software-only GPU #3362: Both modify TerminalWebGLManager.ts — different sections (constant vs new method), no conflict. Merge fix(terminal): reduce WebGL context pool to leave headroom for Chromium limit #3358 first.
• PRs fix(memory): switch to privateBytes and add EMA trend detection #3360 and feat(memory): graduated memory pressure mitigation #3366: Both modify ProcessMemoryMonitor.ts and startAppMetricsMonitor(). Will have git merge conflicts. Merge fix(memory): switch to privateBytes and add EMA trend detection #3360 first, then rebase feat(memory): graduated memory pressure mitigation #3366.
• PRs fix(terminal): reduce default scrollback from 2500 to 1000 lines #3365 and feat(terminal): reduce xterm.js scrollback for background terminals #3361: Both modify scrollback.ts and scrollback tests. Semantic conflict fixed: feat(terminal): reduce xterm.js scrollback for background terminals #3361's SCROLLBACK_BACKGROUND was updated from 1000→500 so it remains meaningful after fix(terminal): reduce default scrollback from 2500 to 1000 lines #3365 lowers the default to 1000. Merge fix(terminal): reduce default scrollback from 2500 to 1000 lines #3365 first, then rebase feat(terminal): reduce xterm.js scrollback for background terminals #3361.
• PRs feat(terminal): reduce xterm.js scrollback for background terminals #3361 and fix(terminal): skip WebGL renderer on software-only GPU #3362: Both modify TerminalInstanceService.ts — different sections (scrollback tier vs GPU state), no conflict.

Recommended merge sequence:

1. fix(terminal): reduce WebGL context pool to leave headroom for Chromium limit #3358, fix(memory): switch to privateBytes and add EMA trend detection #3360, fix(layout): use HollowCircle icon in WaitingContainer to match agent state standard #3359, fix(theme): improve light theme token coverage and expand E2E contrast checks #3364 (independent, any order)
2. fix(terminal): reduce default scrollback from 2500 to 1000 lines #3365 (after group 1)
3. fix(terminal): skip WebGL renderer on software-only GPU #3362, feat(terminal): reduce xterm.js scrollback for background terminals #3361 (after fix(terminal): reduce WebGL context pool to leave headroom for Chromium limit #3358/fix(terminal): reduce default scrollback from 2500 to 1000 lines #3365 respectively, rebase needed)
4. feat(memory): graduated memory pressure mitigation #3366 (after fix(memory): switch to privateBytes and add EMA trend detection #3360, rebase needed)