Standardize "src" quoting in src.mk Makefile targets

[Copilot]

The typecheck and docs-coverage targets in src.mk used unquoted src arguments while other targets (mypy, docs, security) consistently quoted them.

Changes:

• typecheck target: mypy src → mypy "src"
• docs-coverage target: interrogate -vv src → interrogate -vv "src"

All command-line references to the source directory are now consistently quoted throughout the file.

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• astral.sh
  • Triggering command: /usr/bin/curl curl -LsSf REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)