refactor: declare each const in the narrowest scope that holds its uses

[not-elm]

Problem

.claude/rules/rust.md requires each const to live in the narrowest scope that holds its uses, but many constants under src/ and crates/ were still module-level: some named by a single function, others stating a property of one type.

Solution

Apply the rule across src/ and crates/, with no change in behavior.

• Function-local (51): a const that exactly one function names moves to the top of that function's body. Constants that had no doc get a one-line ///.
• Associated (16): a const that states a property of one type moves onto it.

Before	After
FACTORS / BASE	FontZoom::FACTORS / FontZoom::BASE
WEBVIEW_TICK	UpdatePolicy::WEBVIEW_TICK
INITIAL_POOL_CAP	ImeGlyphPool::INITIAL_CAP
MAX_ASSET_LEN	StaticAsset::MAX_LEN
DEFAULT_THICKNESS	CursorConfig::DEFAULT_THICKNESS
DEFAULT_FOREGROUND / DEFAULT_BACKGROUND	Palette::DEFAULT_FOREGROUND / Palette::DEFAULT_BACKGROUND
MAX_TITLE_DEPTH	TitleState::MAX_STACK_DEPTH
MAX_APC_LEN	WebviewApcRequest::MAX_LEN
MAX_INTERMEDIATES	CsiParams::MAX_INTERMEDIATES
MAX_SUBPARAMS	Group::MAX_SUBPARAMS
MAX_INLINE_HTML	ValidatedRegistration::MAX_INLINE_HTML
placement::MAX_ROWS / MAX_COLS	PlacementSize::MAX_ROWS / PlacementSize::MAX_COLS
grid::MIN_COLUMNS	GridSize::MIN_COLS

The last two rows change orzma_vt::prelude: MAX_ROWS, MAX_COLS and MIN_COLUMNS are no longer exported, and MIN_COLUMNS is renamed to match GridSize::MAX_COLS.

Doc comments that named a constant which is now function-local describe the behavior instead.

Associated consts are declared above the methods of their impl, whatever their visibility. .claude/rules/rust.md now states this, and the consts that sat below a method (Binding::LEADER_TOKEN / REPEAT_TOKEN, PointerButton::ALL, QueueSampler::BACKLOG_FLOOR) move up as well.

Left at module level:

• orzma_tty's MAX_CURSOR_KEYS: the type it bounds is pub(crate), and the public WheelConfig doc links to it.

• Constants shared by more than one type (MAX_PLACEMENTS, MAX_COMBINING), and constants named by one function plus a test or another item's intra-doc link.

• src/window_icon.rs is Windows-only and was not compiled locally; CI covers it.

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