Reuse the captured environment for platform theme fallback

[stalep]

Review of terminal-detect, terminal-api, and terminal-tty at 859020a6 on 2026-09-28.

Review item: Performance 3
Priority: P2
Evidence: Repeated detector construction confirmed; end-to-end savings not measured.

Problem / scope

Full detection constructs a TerminalDetector, then detectPlatformTheme()
constructs another one instead of using the captured environment. On Windows
this can repeat registry-based terminal identity subprocesses. It also discards
the injected environment used by a caller/test detector.

Source at the reviewed commit

• terminal-detect/src/main/java/org/aesh/terminal/detect/TerminalCapabilities.java:225
• terminal-detect/src/main/java/org/aesh/terminal/detect/TerminalDetector.java:326

Acceptance criteria

• Reuse the current detector/environment for config and OS theme fallback.
• Count identity subprocess invocations in a fixture and test that the original environment is retained.
• Measure complete startup detection before/after; keep claims separate from environment-only microbenchmarks.

Tracking

Part of #300. Related: #293, #297.