prop-flow: detect props that are always passed the same value

[fxOne]

Summary

prop-flow now reports props that are passed the same value at every call site. Constancy is orthogonal to whether an optional prop's ? earns its keep — a prop can be justified and still be constant everywhere it is passed — so it lands as its own field on PropAnalysis rather than as a sixth verdict. Values are read off the type, not the syntax, so size="sm", a const, an enum member and a property of an as const object all resolve to one value, and a single unreadable site leaves the claim unmade.

Changes

• New constant field ({ coverage, value }) on every PropAnalysis, null where there is no finding. coverage: 'passes' means the passing sites agree; 'all' means the omissions land on the value too via the component's own binding default — the case where the prop can go away entirely. Nothing is reported below two passing sites.
• New values.ts resolves values through checker.getTypeAtLocation plus the literal-type checks, covering string/number/boolean literals, boolean shorthand (<C dense /> is true), enum members and as const properties. A manual site or any type the checker cannot pin to a single literal suppresses the finding.
• --all-props widens discovery to required props, not the report: a required prop appears only when it carries a constant value, under a new required verdict. Without the flag the output is byte-identical to before.
• Binding defaults are now carried as values rather than a boolean. A pass-through level that binds its own default absorbs the omissions at its own call sites and feeds the default down, instead of counting them as omissions at the leaf — which reported an always-set prop as caller-dead. Each absorbed omission is listed as a real site at the call site where the default fires, so the counts still line up with the sites below.
• Usage indexing (indexProgram, JSX tag resolution, the symbol-id allocator) moved out of analyzer.ts into a createUsageIndex module, which is what lets it be tested directly.
• README gains a "Constant values" section, the --all-props row and the absorbed-default example; CHANGELOG cut to 2.0.0 and the version bumped.

Breaking changes in the library API and the --json shape (the CLI arguments and the text output only gained things):

• OptionalProp is now DeclaredProp, with an added optional field
• Analyzer.listOptionalProps is now listProps(component, { includeRequired })
• PropAnalysis.constant is required, so every --json row carries it

Related issues

Closes #934