fix: resolve bounty issue #1950 - posting tags now override inherited txn tags

[bomlinux92-byte]

Summary

Fix for [BOUNTY $20] Inherited Tag Values are not Overwritten — Issue #1950

Root cause

When a posting had a tag with the same name as an inherited transaction-level tag (e.g., concerns: me on transaction, concerns: you on posting), the posting's tag value should overwrite the inherited one. However, postingAllTags was doing a simple concatenation: ptags p ++ txnTags, which meant both values were retained and matched by tag queries.

Fix

Modified postingAllTags in hledger-lib/Hledger/Data/Posting.hs to filter out transaction-level tags whose names conflict with explicitly-set posting tags:

postingAllTags :: Posting -> [Tag]
postingAllTags p = postingTags ++ filteredTxnTags
  where
    postingTags = ptags p
    txnTags = maybe [] ttags (ptransaction p)
    postingTagNames = map fst postingTags
    filteredTxnTags = filter ((`notElem` postingTagNames) . fst) txnTags

This ensures posting-level tags take precedence over inherited transaction tags with the same name, and tag queries like tag:concerns=me only match postings that genuinely have that effective tag value.

Testing

The existing test case a tag match on a posting also sees inherited tags in Query.hs continues to pass (inherited tags without conflicts are still visible), and the fix properly handles the overwrite case.

Closes #1950