feat: add campaign comment system via off-chain attestation (#542)

[bbkenny]

Closes #542

Summary of Changes
Added a campaign comment system using off-chain attestation to keep storage costs low.

• add_campaign_comment: Allows users to post a comment to an active campaign. The function takes an off-chain comment_hash (which could be a CID or text hash depending on the frontend) and emits an event campaign_comment_added without storing the full string on-chain.
• remove_campaign_comment: Gives moderation tools to campaign creators and admins. This flags a specific comment_id as removed using a new CommentRemoved data key and emits a campaign_comment_removed event, signaling the indexer or frontend to hide it.
• Added src/campaigns/comments.rs for the logic and updated errors.rs to track invalid or previously removed comment targets cleanly.

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[davidmaronio]

the design direction here is right: hash-only comments emitted as events with a per-campaign counter and a tombstone key for moderation keeps on-chain storage tiny, and the creator-or-admin removal gate plus double-remove protection is sensible. but the branch does not compile, and a few things look like they were never run locally:

1. src/errors.rs:148-155 - the new variants InvalidCommentId = 46 and CommentAlreadyRemoved = 47 were pasted inside the name() match arm block of impl Error, not into the Error enum itself. this is a syntax error and is why CI is red. move the variants into the enum and add matching arms to name().
2. src/tests/test_comments.rs:5 - the tests use ProofOfHeartContractClient and a setup_env_with_active_campaign(100) helper returning a 5-tuple. please double check those against src/tests/helpers.rs on this branch; other test files in the repo use a different setup shape, and since CI never got past the errors.rs syntax error these tests have never actually compiled or run. please run cargo test --features testutils locally and paste the output.
3. src/campaigns/comments.rs:add_campaign_comment - anyone can spam comments for free apart from tx fees, and each one bumps a persistent counter. that's fine, but consider requiring the campaign to be active (you only check existence) so closed or cancelled campaigns don't keep accepting comments.
4. same file, remove flow - removal only writes a tombstone and emits an event, which matches the off-chain attestation model, but the CommentRemoved tombstone grows one persistent entry per removal forever. acceptable, just worth a comment noting the trade-off.
5. this pr also carries the unrelated test_campaign_update.rs 2-tuple to 4-tuple event assertion change and the trailing-newline touches to the orphaned ProofOfHeartContract blocks, same as your #724/#725/#726. drop the duplicates and let one pr own that cleanup, then rebase the others.

gate: branch is DIRTY and CI is red, so fix the errors.rs compile break, verify the tests actually run, resolve conflicts, and rebase on main.