feat: add DisableCaps() for individual terminal cap control

[mikeschinkel]

Summary

• Add DisableCaps(...Capability) method to TerminalRenderer to allow disabling specific terminal capabilities
• Export capability constants such as CapCBT, CapREP, CapECH, etc. so callers can reference them
• Support UV_DISABLE_CAPS=CBT,REP environment var so end users can adjust for edge-case terminals without code changes

Motivation

xtermCaps() maps TERM values to capability sets, but some terminal emulators report a standard TERM (e.g. xterm-256color) while not correctly implementing all the capabilities that implies. Currently there is no way to selectively disable specific optimizations — the only workaround is overriding TERM to a less capable terminal type, which degrades the entire rendering pipeline (e.g. losing TrueColor).

This PR adds fine-grained control: disable only the problematic capability while keeping everything else at full quality.

Example: JetBrains GoLand and all IntelliJ-based IDEs use JediTerm, which reports as xterm-256color but mishandles CBT (Cursor Backward Tab), causing invalid rendering. A follow-up PR will add auto-detection for known problematic terminals like JediTerm; this PR provides the underlying API that makes that possible, plus a manual escape hatch for terminals not yet auto-detected.

Usage

Programmatic:

renderer.DisableCaps(uv.CapCBT)

Environment variable (useful for debugging):

UV_DISABLE_CAPS=CBT,REP myapp

Changes

• disable_caps.go — New file. Exports Capability type alias, 10 capability constants (CapVPA, CapHPA, CapCHA, CapCHT, CapCBT, CapREP, CapECH, CapICH, CapSD, CapSU), the DisableCaps() method, and applyDisableCaps() for env var support.
• disable_caps_test.go — New file. 4 tests covering single disable, multiple disable, exported constant correctness, and env var parsing.
• terminal_renderer.go — One-line addition: calls applyDisableCaps() during NewTerminalRenderer() so the env var is respected automatically.

Test plan

• TestDisableCaps — single capability disable, others unaffected
• TestDisableCapsMultiple — multiple capabilities disabled simultaneously
• TestExportedCapabilityConstants — exported constants match internal values
• TestApplyDisableCapsEnv — UV_DISABLE_CAPS env var parsing and application
• All existing tests pass