Merge next into issue-4-api-chats (closes #4)

Brings in the calculator's powers and remainders. docs/TODO.md keeps
both Completed Steps lines, the API one first.

Model: opus-5-5

docs/TODO.md

@@ -30,6 +30,9 @@ with no deprecation warning.
 - 2026-09-29 Added the HTTP API in `internal/api`: the server on `PORT`,
   the bearer credential from `API_TOKEN_FILE`, security headers and
   limits, and `GET /api/v1/chats`
+- 2026-09-28 Powers (`^`, also written `**`) and remainders (`%`) in
+  `internal/calc`, which now reads expressions with a parser of its own
+  in place of `go/parser`
 - 2026-09-28 Moved the command tree and the `run` and `version` commands
   from `cmd/simplexcalc/` into `internal/cli`; `cmd/simplexcalc/main.go`
   is now a single call to `cli.Main`