fields: precompile templates into reusable segments

[helly25]

Summary

Precompiles named-field templates so the hot path (rendering every match) stops re-scanning the template string per entry (#51).

• fields::Template — Compile() parses a template once into literal/field segments; Render(path, metadata, depth) walks the segments, appending literals verbatim and resolving fields.
• The free Render(tmpl, ...) now delegates to Compile().Render(), so every existing call site and test is unchanged.
• run.cc compiles the --template once before the walk and renders each match through the compiled form (was: fields::Render(*tmpl, …) re-parsing the string on every matched entry).

Why now

This is the architectural prerequisite for the remaining #51 slices:

• moving field-name resolution to compile time (the constexpr/mbo dispatch table) — the segment already carries a resolved field, so that becomes a Compile-time lookup;
• threading {root} through a render context instead of adding another positional parameter.

No behaviour change.

Test plan

• New CompiledTemplate RendersManyEntries: one Template::Compile reused across two entries, covering a plain field + literal + a qualified field ({size:h}).
• All existing fields_test cases pass unchanged via the delegating free Render.
• Green on both toolchains: bazel test //... (default) and --config=clang --config=asan.