Separate per-connection capability state from process startup defaults

[stalep]

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

Review item: Design 2
Priority: P2
Evidence: Cross-connection shared-state coupling confirmed in call paths.

Problem / scope

Every EventDecoder theme notification calls the process-global
TerminalCapabilities.onThemeChanged(). EventDecoder is also used for remote
SSH/Telnet/HTTP connections. A client's theme can therefore alter global local
defaults, and TerminalFeatures consults shared probe results for per-connection
mode answers. Startup defaults and an individual terminal's state need distinct
ownership.

Source at the reviewed commit

• terminal-api/src/main/java/org/aesh/terminal/EventDecoder.java:655
• terminal-detect/src/main/java/org/aesh/terminal/detect/TerminalCapabilities.java:136
• terminal-api/src/main/java/org/aesh/terminal/TerminalFeatures.java:630

Acceptance criteria

• Define which capabilities belong to an individual Connection and how standalone defaults seed a local connection.
• Test two connections with conflicting themes/modes without cross-contamination.
• Preserve a simple standalone API and decide a compatible migration path for existing consumers.

Tracking

Part of #300. Related: #293, #292.