sbe: a size or id past 65535 is cut to 16 bits instead of refused (protowire-go#254) #132

Every SBE header field is a uint16, and this port narrows a larger value to its low 16 bits with no error. protowire-go refuses each case since trendvidia/protowire-go#254 (issue #250).

What C++ does now (by reading main a632c6e, not measured):

• BuildTemplate (src/sbe/template.cc) stores static_cast<uint16_t>(*tid), so (sbe.template_id) = 65537 is written as 1.
• Codec::New (src/sbe/codec.cc) does the same for (sbe.schema_id) and (sbe.version).
• FieldEncodingSize narrows (sbe.length) with static_cast<uint16_t>, so a 65537-byte field binds as 1 byte.
• Offsets and block lengths are summed in uint16_t in BuildTemplate, BuildGroupTemplate and BuildCompositeFields, so a block past 65535 bytes wraps. In Go this bound and then panicked in Marshal. Here the wrapped length is written.
• Codec::Marshal (src/sbe/marshal.cc) writes numInGroup as static_cast<uint16_t>(n), so a group of 65536 entries is written as 0 entries. It reads back as an empty group with no error.

Go's errors, to mirror:

• sbe: message %s: (sbe.template_id) = %d is past 65535, the largest templateId SBE can state
• sbe: file %s: (sbe.schema_id) = %d is past 65535, the largest schemaId SBE can state (and the same for (sbe.version) / version)
• (sbe.length) = %d is past 65535, the largest blockLength SBE can state
• ends at byte %d of its block, past 65535, the largest blockLength SBE can state, wrapped as sbe: field %s.%s: …, sbe: composite %s.%s: …, sbe: group field %s.%s: …, sbe: composite in group %s.%s: …
• Marshal: sbe: field %s has %d entries, past 65535, the largest numInGroup SBE can state

Go accumulates offsets in int and checks before narrowing, so every offset in a template is in range by construction.

Done when each of these is refused with Go's text, and the same shape at 65535 still binds and round-trips (port encoding/sbe/header_bounds_test.go):

• template_id, schema_id, version past 65535
• (sbe.length) past 65535
• a message block, group entry block and composite past 65535 bytes
• Marshal of a group with more than 65535 entries

Note: #124 (stale-descriptor diagnosis) also edits Codec::New and RegisterMessage, so this work should stack on it or follow its merge.