v0.3.1 compliant, non-accept after challenge, duplicate before continue

[markmnl]

No description provided.

[Copilot]

Pull request overview

Updates host-side protocol flow to better align with the v0.3.1 spec by moving some response decisions out of downloadMessage and into the initial-accept phase in handleConn.

Changes:

• Refactors add-to handling so handleAddToPath sets InitialResponseCode and defers sending/storing to handleConn.
• Removes the post-accept global-duplicate response path and introduces a pre-continue duplicate check in handleConn.
• Updates spec text and header field comment to include accept code 11 in InitialResponseCode.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/host.go	Moves add-to response/send/store into handleConn; removes respondGlobalDuplicateIfNeeded; adds duplicate check before sending code 64.
src/host_test.go	Removes unit test for respondGlobalDuplicateIfNeeded.
src/defs.go	Updates InitialResponseCode comment to include code 11.
SPEC.md	Reworks receiving flow description and minor formatting tweaks.

[Copilot]

This PR removes the only unit test that exercised the global duplicate response behavior, but does not add coverage for the new behavior (duplicate check now happening in handleConn before sending the initial accept code). Please add/adjust tests to cover at least:

• global duplicate rejection is sent as the initial response (code 10) when all local recipients already have the challenged message hash
• this holds for both the continue (64) and skip-data (65) paths, if those flows are intended to support global-duplicate rejection.

Suggested change

	}
	}
	func TestHandleConnRejectsGlobalDuplicateAsInitialResponseOnContinuePath(t *testing.T) {
	runHandleConnGlobalDuplicateInitialResponseTest(t, 64)
	}
	func TestHandleConnRejectsGlobalDuplicateAsInitialResponseOnSkipDataPath(t *testing.T) {
	runHandleConnGlobalDuplicateInitialResponseTest(t, 65)
	}
	func runHandleConnGlobalDuplicateInitialResponseTest(t *testing.T, expectedPath byte) {
	t.Helper()
	host, conn, done := newHandleConnTestHarness(t)
	t.Cleanup(func() {
	conn.Close()
	<-done
	})
	req := newChallengedMessageRequest(t)
	seedAllLocalRecipientsWithChallengedHash(t, host, req)
	go handleConn(conn)
	writeHandleConnRequest(t, conn, req, expectedPath)
	got := readHandleConnResponseCode(t, conn)
	if got != 10 {
	t.Fatalf("expected initial global duplicate reject code 10 on path %d, got %d", expectedPath, got)
	}
	}