Serve large installer downloads without a write deadline

[SaladDay]

What

• Core clears the response write deadline before it serves a local native installer archive (D5). The server-wide 30 s WriteTimeout stays for everything else. Node artifacts and the Runtime image are served by the Web service, which has no write timeout; the other Core streams already set bounded deadlines.
• When claiming the Session's credential returns 409, the installer removes the executor-credential.json it wrote. The message says the Session's install command cannot be used again and that Rotate plus an extracted install, or a new Session, reconnects it (D6).
• The bootstrap accepts a complete .part after verifying its checksum, so curl no longer prints the 416 line (D7).
• The bootstrap prints Interrupted; rerun the same command to continue. on Ctrl-C (D8).

Why

Audit findings D5-D8 from the live-injection run.

Verification

• go test ./services/core/internal/nativeinstaller ./apps/daemon/internal/cli: pass. The new write-deadline test fails without the fix.
• go vet on both packages and scripts/check-names.py: pass.

Not done

• D6 Web state: Core's Session read does not say the credential is claimed (the install status is available), so Web cannot show it without a new field. The Executor credentials list exists but cannot tell a claimed key from an issued one, and hiding the command would remove the documented rerun in the original directory. Left out.
• D8 for the PowerShell bootstrap: left unchanged.

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