runtime coverage: add byte-offset lookup helpers for V8 coverage pipelines

[BartWaardenburg]

Why

Runtime coverage pipelines often start with V8 byte offsets, not line/column pairs.

Fallow Runtime currently has to normalize V8 offsets into line/column positions before source-map lookup. That logic belongs close to the source-map SDK because correctness depends on the same details srcmap already owns:

• generated byte offsets vs generated line/column positions
• UTF-8 byte offsets vs UTF-16 JavaScript columns
• missing/generated-only positions
• bundled/minified output with sparse mappings
• line/column APIs that are already 0-based internally

The existing public APIs are strong for original_position_for(line, column) and generated_position_for(source, line, column), but runtime coverage callers need a first-class path from generated byte offsets to original source locations.

Scope

Add helper APIs for generated byte-offset lookup.

Possible API direction:

pub struct GeneratedOffsetLookup<'a> { ... }

impl GeneratedOffsetLookup<'_> {
    pub fn new(generated_source: &str) -> Self;
    pub fn byte_offset_to_position(&self, byte_offset: u32) -> Option<GeneratedPosition>;
    pub fn original_position_for_offset(&self, sm: &SourceMap, byte_offset: u32) -> Option<OriginalLocation>;
}

Alternative shapes are fine; the important bit is that runtime coverage consumers do not each reimplement offset normalization.

Acceptance criteria

• Supports UTF-8 source text and documents what column unit is passed into source-map lookup.
• Tests cover ASCII, multi-byte UTF-8, surrogate-pair JavaScript columns if relevant, CRLF, and offsets at line boundaries.
• Works with both eager SourceMap and, if feasible, LazySourceMap.
• Example shows V8 function range start offset -> original TypeScript file/line/column.
• README documents the runtime coverage use case.

Non-goals

• Do not build a full V8-to-Istanbul converter in srcmap.
• Do not take a dependency on Fallow-specific protocol types.
• Do not change existing line/column lookup semantics.

[BartWaardenburg]

Primary downstream cloud consumer: https://github.com/fallow-rs/fallow-cloud/issues/63