perf(render): fix lazy-render and fold performance for large sessions

[jensenojs]

Problem

2786-message sessions take ~36 seconds to render. Two root causes:

1. set_folds() uses cursor() + normal! zc/zo per fold, each triggering a full-screen redraw (~90ms/fold × 392 folds)
2. _render_full_session_data() renders all messages on initial load

Closes #392

Changes

Fold performance (36s → 3s)

• Replace vim.fn.cursor() + normal! zc/zo with :{from},{to}fold and :{from},{to}foldopen! — Ex commands don't move the cursor, zero screen redraws
• Switch to foldmethod=manual before creating folds and stay there — prevents foldexpr recalculation for every buffer line (~4.6s on 87K lines)
• Add line count validation to prevent E16: Invalid range errors

Lazy render (3s → ~10ms)

• Only render enough messages to fill the output window on initial load (~1.5× viewport height)
• Load more on scroll-to-top via WinScrolled autocmd
• opts.lazy = true explicit opt-in — tests and headless environments render all messages
• Fix lazy_render_count being cleared by M.reset(): read before reset, persist back to ctx after determining limit
• Debounce WinScrolled load_more callback (150ms) to prevent rapid re-renders during fast scrolling
• Remove debug vim.notify logging from render paths

Performance

Phase	Before	After
Fold creation	~35,266ms	~0.8ms
Total render	~36,000ms	~10ms
E2E (incl. async fetch)	~36,000ms	~1,875ms

Measured on a 2786-msg / 87K-line session

Testing

7 new test cases in tests/unit/renderer_lazy_spec.lua:

• Renders all messages when lazy=false
• Renders limited messages when ctx.lazy_render_count is set
• Preserves lazy_render_count increment across render reset (exposes the core bug)
• load_more_messages increments rendered message count
• load_more_messages returns false when all/no messages loaded
• No INFO-level debug notifications during rendering

All 110 existing tests pass.

[sudo-tee]

Thanks for the PR.

I will have a look at soon, I will be out for the weekend. So I will look at it at the start of the next week

[jensenojs]

I want to go out and have fun too. Have a nice weekend!