Add viewport info to JSON UI draw message

[Yus314]

Summary

Append first_buffer_line, last_buffer_line, and buffer_line_count parameters to the draw JSON-RPC method, enabling alternative frontends to implement scroll bars, minimap indicators, and other viewport-aware UI elements.

This is the viewport information subset extracted from the scroll bar PR (#5304), proposed as an independent protocol extension that benefits all JSON UI consumers regardless of whether a built-in scroll bar is added.

Motivation

Alternative frontends (e.g. Kasane) currently cannot determine the buffer position corresponding to the displayed viewport. The information needed — which buffer lines are visible and how many total lines exist — is available inside Kakoune but not exposed through the JSON-RPC protocol. Without these values, features like scroll bars, minimap position indicators, and viewport-relative overlays require fragile heuristics that break with word-wrap, replace-ranges, and folding.

Implementation

New parameters (appended to existing draw positional params):

Parameter	Type	Source
first_buffer_line	int (0-based)	window.last_display_setup().first_line
last_buffer_line	int (0-based)	display_buffer.lines().back().range().begin.line
buffer_line_count	int	context.buffer().line_count()

Data source rationale:

• first_buffer_line uses last_display_setup().first_line rather than display_buffer.range().begin.line because the latter is computed before highlighter passes and becomes stale after wrap/trim/erase operations.
• last_buffer_line uses the last display line's range begin, which is updated by compute_range() during trim_from and reflects the actual last visible buffer line after all highlighter transforms.

Files changed:

• src/user_interface.hh — Added 3 LineCount params to draw() virtual interface
• src/client.cc — Extract and pass viewport values from correct sources
• src/json_ui.cc/hh — Added to_json(LineCount) overload; updated draw signature and rpc_call
• src/terminal_ui.cc/hh — Updated signature (values unused, unnamed params)
• src/remote.cc — Updated signature; send_message/template dispatch handles serialization automatically
• src/main.cc — DummyUI draw override updated
• doc/json_ui.asciidoc — Protocol documentation updated
• 74 test script files — Updated draw message assertions with viewport values

Backward compatibility: Existing JSON UI consumers that don't parse trailing positional parameters are unaffected. This follows the same "append to end" pattern established by widget_columns (#5455) and draw_status style (#5458).

Test plan

• make builds with zero new warnings
• All 74 test scripts with draw messages updated with correct viewport values
• make test passes (656 tests, 0 failures)
• Manual verification: kak -ui json emits correct viewport values with word-wrap, replace-ranges, and large buffers

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Discussed in #5304 (comment)