Support high-bit-zero numbering on bits types

[Copilot]

Many big-endian protocols (especially older RFCs like IP/TCP) number bits with bit 0 as the MSB. This adds a bit_numbering attribute to support that convention.

Changes

• New bit_numbering attribute: Accepts "Lsb0" (default) or "Msb0" on bits types and as module default
• C++ runtime: Added BitBlockMsb0 and OffsetBitBlockMsb0 classes that transform user offsets to physical offsets via physical = total_bits - user_offset - size
• Code generator: Routes to BitBlockMsb0 when the referenced bits type has [bit_numbering: "Msb0"]
• Documentation: Updated language reference with examples

Example

[$default byte_order: "BigEndian"]

bits IpFlags:
  [bit_numbering: "Msb0"]
  0 [+1]  Flag  reserved      # Bit 0 = MSB
  1 [+1]  Flag  dont_fragment
  2 [+1]  Flag  more_fragments
  3 [+13] UInt  fragment_offset

struct IpHeader:
  0 [+2]  IpFlags  flags_and_offset
    [byte_order: "BigEndian"]

This matches RFC 791's bit numbering convention directly.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• www.googleapis.com
  • Triggering command: /usr/local/lib/node_modules/@bazel/bazelisk/bazelisk-linux_amd64 /usr/local/lib/node_modules/@bazel/bazelisk/bazelisk-linux_amd64 build //... (dns block)
  • Triggering command: /usr/local/lib/node_modules/@bazel/bazelisk/bazelisk-linux_amd64 /usr/local/lib/node_modules/@bazel/bazelisk/bazelisk-linux_amd64 --version (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support High-Bit-Zero Numbering on bits</issue_title>
<issue_description>This issue was copied from the upstream repository google/emboss.

Original issue: google#42
Original state: open, created at: 2022-04-20T03:17:02Z, updated at: 2024-12-18T23:14:43Z, by @reventlov

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Original description

A number of big-endian protocols number bits with bit 0 as the highest-order/first-read bit. This seems to be particularly common in protocols that are expressed in terms of bitstreams.

Emboss should provide a way to allow bits to be addressed with bit 0 as the highest-order bit.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Support High-Bit-Zero Numbering on bits #81

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