Context

Implements PR 3a of the per-entry changelog plan (see #4498, #4499, and plans/per-entry-changelog-4498.md). This is the read-path half of "Toolshed PR 3"; the write-path half (write_version / write_current / write_date / changes_for_commit) lands separately as PR 3b.

Dependencies landed:

• py/envoy.base.utils: Add per-entry RST directory reader #4518 — AChangelog.get_data_from_entries(yaml_path, entry_dir) exists.
• py/envoy.base.utils: Switch to runtime validation for changelog sections #4519 — AChangelogs.validate_sections(data, path=None) exists and is called from AChangelog.data.

Backward-compatibility principle (non-negotiable)

Detection is a single is_dir() check against <project_path>/changelogs/current. No config flag, no version pin, no deprecation timeline. Both layouts must be supported indefinitely so that:

• Stable release branches (release/v1.xx) cut before the migration keep working unchanged.
• Cherry-picks to legacy branches keep building.
• Toolshed wheels can be upgraded on legacy branches without breakage.

If changelogs/current/ is a directory → entries layout. Otherwise → legacy layout. Implementation must be strict: the entries path must never run on a repo without current/, and the legacy path must never look at current/.

What this PR does

File: py/envoy.base.utils/envoy/base/utils/abstract/project/changelog.py

1. Layout-detection helper on AChangelogs

Add a property:

@property
def _entries_layout(self) -> bool:
    """Whether the project uses the per-entry changelog layout.

    Detected by the presence of the `changelogs/current/` directory.
    A single `is_dir()` check; no caching beyond what `pathlib` does.
    """
    return self.project.path.joinpath(CHANGELOG_CURRENT_DIR_PATH).is_dir()

Match the existing file's style (other properties on AChangelogs are @property or @cached_property). Do not cache — the directory may appear/disappear during a single process lifetime (e.g. write_version removes it). If profiling shows is_dir() is hot, a future PR can add a @cached_property with explicit invalidation; not this PR's problem.

2. Per-entry directory path helper on AChangelogs

Add a property that returns the entries directory path (parallel to current_path / rel_current_path):

@property
def current_dir_path(self) -> pathlib.Path:
    return self.project.path.joinpath(self.rel_current_dir_path)

@property
def rel_current_dir_path(self) -> pathlib.Path:
    return pathlib.Path(CHANGELOG_CURRENT_DIR_PATH)

Mirrors the existing current_path / rel_current_path pair exactly. These will be reused by PR 3b; introducing them now keeps the read-path PR self-contained.

3. Dispatch AChangelog.get_data / AChangelog.data on layout

Current state (post-#4519):

@classmethod
def get_data(cls, path) -> typing.ChangelogDict:
    try:
        data = utils.from_yaml(path, typing.ChangelogSourceDict)
    except (_yaml.reader.ReaderError, utils.TypeCastingError) as e:
        raise exceptions.ChangelogParseError(f"Failed to parse: {path}\n{e}")
    return cast(
        typing.ChangelogDict,
        {k: (v if k == "date" else [...]) for k, v in data.items() if v})

@async_property(cache=True)
async def data(self) -> typing.ChangelogDict:
    return self.project.changelogs.validate_sections(
        await self.project.execute(self.get_data, self.path),
        self.path)

AChangelog.get_data is a @classmethod — it has no access to AChangelogs, so it cannot do layout detection itself. The dispatch must happen in AChangelog.data (which has self.project.changelogs).

Change data to detect layout and dispatch:

@async_property(cache=True)
async def data(self) -> typing.ChangelogDict:
    changelogs = self.project.changelogs
    if self.is_current and changelogs._entries_layout:
        parsed = await self.project.execute(
            self.get_data_from_entries,
            self.path,
            changelogs.current_dir_path)
    else:
        parsed = await self.project.execute(self.get_data, self.path)
    return changelogs.validate_sections(parsed, self.path)

Key constraints on this:

• Gate on self.is_current. The per-entry layout only applies to the current changelog. Historical <version>.yaml files keep using the legacy reader regardless of whether current/ exists. AChangelog has access to its version via self.version, and AChangelogs already has an is_current notion (in AChangelogStatus on the checker side; on the aggregator side, current is next(iter(self.changelogs))). The cleanest check is: self.version == changelogs.current. Use whichever spelling already exists in this file's style; if there isn't one, add a small private _is_current property on AChangelog.

• validate_sections still runs for both branches. It's source-agnostic and already exists on AChangelogs.

• The classmethod get_data is unchanged. Do not touch its body. get_data_from_entries is unchanged. Only the dispatch in the `...