fix: address mapProps review points (#1-#4, B1, B2)

[AlexandrHoroshih]

Follow-up to PR #102 (mapProps feature). Addresses all review points:

#1 — Runtime tests added for variant, list, createReflect (no-ssr + ssr).
Previously only reflect had mapProps coverage.
#2 — Unknown mapProps keys now error at the key site itself, not on fn's
return. MapPropsFromSources resolves unknown-key entries to never,
producing a TS2322 at the key line. Replaces the old
'K extends keyof Props ? Props[K] : never' fn-return approach that
had a silent-failure path (never-returning fn compiled cleanly).
#3 — Documented the Store variable-source widening limitation.
Type tests (3a positive, 3b widening) pin the current behavior.
#4 — fn is skipped when its key is overridden by an external prop.
'if (key in props) continue' in src/core/reflect.ts. Spy assertions
in reflect and createReflect tests verify fn is not called.
B1 — bind + mapProps key collision is now a type error. MapPropsFromSources
takes Bind as a type parameter; a key in both bind and mapProps
resolves to never. Runtime skip ('if (key in storeProps) continue')
is a defense-in-depth for JS/type-bypass scenarios (covers stores;
events/data/functions are covered by the type fix).
B2 — mapItem + mapProps key collision in list is a type error. MapItem's
mapped type now omits keyof Sources alongside keyof Bind. Partial:
bypassable with explicit item-parameter annotation (known TS
limitation with mapped-type extends constraints), documented in the
type-test comment.

All four operators (reflect, createReflect, list, variant) are covered by the type fixes and runtime tests.

Docs updated in docs/pages/docs/reflect.mdx:

• mapProps keys must be props of the view; unknown keys error at the key
• a key must not appear in both bind and mapProps
• in list, a key must not appear in both mapItem and mapProps
• source should be an inline literal for best inference
• fn is not invoked when its key is overridden

80 runtime tests + type tests pass.

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