feat(format): support field selectors in interpolation

[thiremani]

Problem

String interpolation markers currently resolve only a single identifier. As a result, -p.name interpolates p and leaves .name as literal text, even though field selection is valid in normal expressions.

Desired behavior

Field selectors should be part of the interpolation marker:

• -p.name interpolates the value of p.name.
• -p.age%6d interpolates p.age using the explicit %6d format.

Scope

• Parse one or more .field selectors after the marker root.
• Resolve them with the same dot-expression semantics used outside strings.
• Apply any explicit format to the selected value.
• Report invalid or unknown selectors as compile errors rather than partially consuming the marker.
• Specify interactions with escaped markers and undefined names.
• Add compiler and end-to-end coverage for default and explicit formatting.
• Update the formatting documentation.

This should follow PIR so interpolation expressions participate consistently in validation, dependency/range discovery, and lowering. The implementation should reuse ordinary field-access behavior rather than introduce interpolation-only lookup rules.