[VOTE] Reserve main as the default branch alias

[majin1102]

The implementation PR is here: #7180

This thread is for voting only. Please keep detailed discussion in the PR.

Proposal

Reserve main as the user-facing alias for the default branch.

In reference / checkout contexts, "main" refers to the default branch and is equivalent to an absent branch name / None.

In branch management contexts, "main" is reserved and cannot be used as a real branch name.

Spec Changes

Clarify the branch/tag format spec:

• main is reserved for the default branch.
• The default branch is represented by null / absent branch metadata.
• No _refs/branches/main.json is created.
• parentBranch and tag branch fields use null / absent values for the default branch.

There is no protobuf schema change and no migration required.

Compatibility

Existing datasets and existing default-branch references are unaffected. This only makes the main alias and reserved-name behavior explicit and consistent across APIs.

Vote

Per docs/src/community/voting.md, this is a Lance Format Specification modification: 3 binding +1 votes from PMC (excluding the proposer), 1-week minimum voting period.

Please vote with one of the following:

• +1 approve
• 0 abstain / no strong opinion
• -1 do not approve

If you vote -1, please include the blocking concern.

[jackye1995]

Do we want to do case sensitive or insensitive?

[majin1102]

Currently this is case-sensitive: only exact "main" is special.

I lean toward keeping it that way to match Git/GitHub ref semantics and avoid adding a Lance-specific case-insensitive rule. What do you think