Write product comparison reports as UTF-8

[Nitjsefnie]

Related issue

Refs #627

What does this change?

Makes the XML and PDF product report writers use explicit UTF-8 so comparison output containing Unicode markers can be saved independently of the host's default text encoding.

• Adds route-level subprocess coverage under a verified non-UTF-8 default and requires exact UTF-8 bytes, including → and ⚠.
• Covers the two product report writers only. Developer and demo writers remain follow-up scope in issue 627.
• Integration note: draft PR feat(cli)!: return the canonical diff document from --format json (#693) #702 overlaps both product writer functions; if it lands first, its resulting writers must preserve the explicit UTF-8 arguments.

How to test

• Targeted candidate run 32806936742, job 97678782199: both product-route tests passed on 0f94e3aba052d0ed4976da0767475d2a16742e8f.
• Four single-writer mutation runs reintroduced encoding-less XML/PDF descriptor writers after closefd=False lifecycles and through byte-spelled paths. Each failed only its corresponding behavior assertion under ANSI_X3.4-1968; the sibling route passed: 32807070688, 32807148081, 32807219847, 32807291945.
• Two behaviorally equivalent explicit-UTF-8 descriptor controls both passed: 32807369943 and 32807441424.
• Full fork CI 32808250584: all 27 jobs passed; every checkout-bearing job logged candidate SHA 0f94e3aba052d0ed4976da0767475d2a16742e8f.
• Independent adversarial review: SHIP 0f94e3aba052d0ed4976da0767475d2a16742e8f.

Checklist

• Linked the issue above (Closes #...)
• Ran the CI gates locally and they pass (see What CI checks)
• New or changed behavior has tests
• For a bug fix: the test fails without the fix, and I ran it both ways to check
• Disclosed AI assistance below, if any

AI assistance

GPT-5.6 Luna implemented and tested the change. GPT-5.6 Sol independently reviewed and verified it.

Generated by GPT-5.6 Luna (implementation, testing), GPT-5.6 Sol (brief, review, verification)

[willhea]

Looks great, thanks @Nitjsefnie!