refactor(sequencer): align streaming Inbox selection names with their docs

[spalladino]

Renames the streaming Inbox selection functions so their names use the words their own docs, tests and README already use, and fixes one variable that named the wrong concept. No behaviour change. Stacked on #340.

Context

The selection module called the greedy step "ordinary" and the endpoint-lookup threshold a "ceiling", while every comment, test and README paragraph said "greedy" and "threshold". The node's simulator stored the result of selectSafeLocalEnd in a variable named greedyEnd, which is the other function's concept. The three observed counts every selection function takes were declared inline four times with no docs.

Approach

• getOrdinaryCeiling → getEndpointLookupThreshold, selectOrdinaryMessageEnd → selectGreedyEnd.
• The simulator's local and its debug log key become safeLocalEnd.
• New InboxSelectionPositions type (cursor, local synced, checkpoint start counts) with a doc per field, used by the selection functions and the consumption class's snapshot. It is structurally identical to the inline types, so no call site changes.
• A short module header states what no single function owns: the shared cumulative-count coordinate, its three suffix spellings, and that bucketSeq is an ordinal, not a count.
• README: corrects the description of the endpoint lookup bound, which for a non-final block is the checkpoint-wide reach, not the block's.

API changes

getOrdinaryCeiling, selectOrdinaryMessageEnd and mustQueryEndpoint are removed from the @aztec-labs/sequencer-client root export. Nothing outside the package imports them, and they have only shipped in v6 nightlies.