Preserve pre-untyped stats spec points

This attempts to minimise the changes of a731d12, to avoid unnecessary spec
point churn and make the 2.1.0...3.0.0 diff more readable.

I’m assuming the introduction of TS15 was done to avoid having a load of
deletion notices for the TS12d-o subpoints, which is a valid aim, but I think
overall it’s better this way.

textile/features.textile

@@ -1384,14 +1384,27 @@ h4. Stats
 
 * @(TS1)@ @Stats@ is a type encapsulating a statistics datapoint retrieved from the "REST stats endpoint":/rest-api/#stats.  See "example statistics in JSON format":/general/statistics/
 * @(TS2)@ This clause has been deleted. It was valid up to and including specification version 2.1.
-* @(TS15)@ The attributes of a @Stats@ object consist of:
-** @(TS15a)@ @intervalId@ (property present in the JSON) - a @String@
-** @(TS15b)@ @unit@ (property present in the JSON) - a @String@ or (if idiomatic) a value of the enumerable type @StatsIntervalGranularity@ whose permitted values are @minute@, @hour@, @day@, and @month@. This must be from the @unit@ property of the JSON, not calculated from the @intervalId@
-** @(TS15c)@ @inProgress@ (property present in the JSON) - an optional @String@ containing the last sub-interval included in this entry (in format yyyy-mm-dd:hh:mm) for entries that are still in progress, such as the current month
-** @(TS15d)@ @entries@ (property present in the JSON) - a @Dict<String, int>@ containing statistics entries
-** @(TS15e)@ @schema@ (property present in the JSON) - a @String@ containing the JSON schema URI
-** @(TS15f)@ @appId@ (property present in the JSON) - a @String@ containing the ID of the Ably application the statistics are for.
-** @(TS15g)@ @intervalTime@ - a language-idiomatic @Time@ object, parsed from the @intervalId@
+* @(TS12)@ The attributes of a @Stats@ object consist of:
+** @(TS12a)@ @intervalId@ (property present in the JSON) - a @String@
+** @(TS12b)@ This clause has been replaced by "@TS12p@":#TS12p. It was valid up to and including specification version 2.1.
+** @(TS12p)@ @intervalTime@ - a language-idiomatic @Time@ object, parsed from the @intervalId@
+** @(TS12c)@ @unit@ (property present in the JSON) - a @String@ or (if idiomatic) a value of the enumerable type @StatsIntervalGranularity@ whose permitted values are @minute@, @hour@, @day@, and @month@. This must be from the @unit@ property of the JSON, not calculated from the @intervalId@
+** @(TS12q)@ @inProgress@ (property present in the JSON) - an optional @String@ containing the last sub-interval included in this entry (in format yyyy-mm-dd:hh:mm) for entries that are still in progress, such as the current month
+** @(TS12r)@ @entries@ (property present in the JSON) - a @Dict<String, int>@ containing statistics entries
+** @(TS12s)@ @schema@ (property present in the JSON) - a @String@ containing the JSON schema URI
+** @(TS12t)@ @appId@ (property present in the JSON) - a @String@ containing the ID of the Ably application the statistics are for.
+** @(TS12d)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12e)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12f)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12g)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12h)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12i)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12j)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12k)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12l)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12m)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12n)@ This clause has been deleted. It was valid up to and including specification version 2.1.
+** @(TS12o)@ This clause has been deleted. It was valid up to and including specification version 2.1.
 * @(TS3)@ This clause has been deleted.
 * @(TS4)@ This clause has been deleted. It was valid up to and including specification version 2.1.
 * @(TS5)@ This clause has been deleted. It was valid up to and including specification version 2.1.
@@ -1400,7 +1413,6 @@ h4. Stats
 * @(TS8)@ This clause has been deleted. It was valid up to and including specification version 2.1.
 * @(TS9)@ This clause has been deleted. It was valid up to and including specification version 2.1.
 * @(TS10)@ This clause has been deleted. It was valid up to and including specification version 2.1.
-* @(TS12)@ This clause has been deleted. It was valid up to and including specification version 2.1.
 * @(TS11)@ This clause has been deleted. It was valid up to and including specification version 2.1.
 * @(TS13)@ This clause has been deleted. It was valid up to and including specification version 2.1.
 * @(TS14)@ This clause has been deleted. It was valid up to and including specification version 2.1.
@@ -2190,16 +2202,16 @@ class ConnectionStateChange: // TA*
   reason: ErrorInfo? // RTN4f, TA3
   retryIn: Duration? // RTN14d, TA2
 
-class Stats: // TS15
-  intervalId: String // TS15a
-  unit: Stats.IntervalGranularity // TS15b
-  inProgress: String? // TS15c
-  entries: Dict<String, Int> // TS15d
-  schema: String // TS15e
-  appId: String // TS15f
-  intervalTime: Time // TS15g (calculated client-side)
+class Stats: // TS12
+  intervalId: String // TS12a
+  intervalTime: Time // TS12p (calculated client-side)
+  unit: Stats.IntervalGranularity // TS12c
+  inProgress: String? // TS12q
+  entries: Dict<String, Int> // TS12r
+  schema: String // TS12s
+  appId: String // TS12t
 
-enum StatsIntervalGranularity: // TS15c
+enum StatsIntervalGranularity: // TS12c
   MINUTE
   HOUR
   DAY