V-L3-H1: span-aware TOML parse errors (filename:line:col)

[hyperpolymath]

Context

TOML parse errors come back as anyhow::Context strings. The toml crate supports span-aware errors (toml::de::Error::span()).

What to do

Replace the with_context("Failed to parse manifest") chain with explicit span extraction and a VerisimError::ManifestParse { path, line, col, msg } variant.

Acceptance

• Error output includes filename:line:col
• unit test on a malformed manifest