error codes: migrate parser.rs to PRS-0xx codes #358

Background

Part of #277's rollout — the Foundation PR (#357, merged) built the MinigrafError/ErrorCategory/REGISTRY/bail_coded!/err_coded! machinery with zero call sites migrated. This issue is step 2 of that rollout: migrate src/query/datalog/parser.rs to real PRS-0xx codes (79 of the 130 documented codes in docs/ERROR_REFERENCE.md — the largest category).

Blocker to resolve first (discovered during #357's final review)

src/query/datalog/parser.rs is not anyhow-based — every parser function, including pub fn parse_datalog_command(input: &str) -> Result<DatalogCommand, String> (118 Err(...) returns across the file), returns Result<_, String>. bail_coded!/err_coded! expand to Err(anyhow::Error::new(CodedError::new(...))), which is a type error inside any Result<_, String>-returning function — they are not a drop-in replacement here the way they are in every other module.

Compounding it: src/db.rs's three call sites (Minigraf::execute, WriteTransaction::execute, Minigraf::prepare) each do parse_datalog_command(input).map_err(|e| anyhow::anyhow!("{}", e))? — even if the parser produced a coded error some other way, this stringifies it via Display, destroying the chain at exactly the boundary MinigrafError::from(anyhow::Error) depends on for code detection.

Must decide before planning this issue in detail — two options (see docs/superpowers/specs/2026-08-25-structured-error-codes-design.md, Migration phasing, step 2, for full writeup):

• (a) Change parser.rs's error type to anyhow::Error (or a CodedError-carrying type) and drop the three stringifying map_err calls in db.rs. Simpler, keeps one mechanism crate-wide.
• (b) Add a parallel String-flavored code-tagging mechanism for parser.rs specifically. Avoids touching the parser's error type but permanently duplicates the tagging machinery.

Work item

Once the blocker is resolved: migrate all parser.rs call sites to carry their real PRS-0xx code (via bail_coded!/err_coded! or the chosen equivalent), and add regression tests asserting the specific code comes back from db.execute()/db.prepare() for each.

Depends on: #357 (merged)
Part of #277